It could be compatibility or other reasons such as money issues. You would have to go by a case by case basis but the reasons can vary from couple to couple. If the couple is abusive the family doesn't like the partner etc., 

I agree with Mattaya above but I would like to add one item that I have observed and that is : major and unresolved cross-cultural differences.  I have seen this happen again and again and again -- expectations on one part or on the other that are just stifled by cultural impediments.  Additionally, perhaps the foreign spouse cannot adjust to China in the end or the Chinese spouse cannot adjust to overseas life.

Cultural differences are an issue for sure.
But I would have to say communication issues (or lack of communication) because of language are a major reason.
No matter how well each person speaks English or Chinese there are always things lost in translation that get understood in the wrong way.  This can cause no end of grief at times.

I agree with thedude. Communication is at the root of all true relationships, and sometimes, very small things, like misunderstanding one word, can lead to misunderstanding a whole idea. My wifes English is a world better than my Chinese, so we primarily talk in English. I wish this wasn't so, and I'm making efforts to learn Chinese, because, let's face it, some concepts are very hard to translate. When she finds it difficult to put thoughts into English, it would really help her if I knew her language. 

One has to be extremely patient, forgiving, and show a large degree of humility from time to time. Same culture relationships are hard enough, but throw into the mix being from two cultures as different as East and West, and "Yikes!"

But, hey, I wouldn't trade her for the world!

To me there is not only one reason, but a combination of them.  The Dude pointed out two already that I do consider to be main causes, but to those two I will add "priorities". 
And by "priorities" I do mean that the "laowai" gets married mostly because he is really in love, while the Chinese gets married for many reasons besides "love".  A desire for a better life, a desire for emigrating eventually, financial security during elder years, and yes, sometimes is "love" too, mixed with parts of the others mentioned.

Maybe in a way, "laowais" are a bit more idealistic, while locals are more "materialistically" inclined.

communication: regardless of language barriers. many locals dont beielve in being upfront and honest anbout what they want or what they are thinking. they are always trying to out smart you. 

In my experience with the ladies here I've found that many Chinese girls are really childish. Most foreign men with a backbone won't tolerate it for long. No idea about the foreign girl/Chinese man couples. Lack of communication probably. 

Pre-conceived expectations. Years of Hollywood and films created a mystic Chinese persona to be docile, compliant and submissive.In the most recent memory Laowai are thought to be advanced in technology, and were presumed to be well off. In reality, we are all the same human being wherever and whatever our specific circumstances are. Our human weakenesses aoemtime prevails and relationship did not go past the ifrst year honeymoon period of living together. Don't get me wrong there are many exception whether it is happening in China or in Canada. I see many inter-racial relationships blossomed. Ultimaltely it is tolerance, understanding and most of all love and a stable and comfortable financial independence of both parties that makes the difference...in my opinion.

I am back in the States for the Chinese New Year, and brought up this topic to the wife of a friend of mine who is a marriage counselor. This is what she told me:
Many mixed marriages are succesful precisely because the couple know that there will be problems in the future so they are "bracd" to deal with them with the result being that they end up being more patient and communicate more before "blowing up". Same culture couples, often because they are of the same culture subcoucioulsy assume that their mate "knows or should know" what is on their mind and as such do not communicate clearly.
Her bottom line is that in order to have a succesful marriage/partnership you need clear communication, patience and respect no matter who or where the couple comes from.

Some relationships go well, until you find out the girl's already married!

Chinese women lack patience and have only idealistic thoughts, they do not think outside of what they can get their hands on. PERIOD

When she finds out that the I-Pad you got her was really a clone!

From my Experience they get sick of have to use a wheelchair every morning because of ever night.

Easy , Mothers - in - Law  99% seems to be from what I have been told , telling their daughter in law , what wonderful son she has and that she is not looking after him well enough 

good question!

the reason is: the foreign man doesn't love the Chinese girlfriend/wife enough--"he is just not that into her".  (i don't know the case for foreign woman & Chinese man)

because all my foreign friends & customers (men) who marry Chinese(women)  are located in the women's hometown or living city. and, e.g. a man quitted everything in his home country(Holland ),he has 6 sisters and no brothers. but he still came to China and married his Hangzhou wife......sometimes he complained this and that about the wife to his friends also, but he still can tolerate and stand all of the "culture differences""language obstacles" ,etc .so they married for 3 years,and now his wife is pregnant and everything runs better and better...

 therefore, i think, the couple break up, because the man doesn't love the woman enough,otherwise, he will always find her!

-As the relationship progressed my gf was taking more and more control over my life, first I had to stop talking to one girl (former student was a friend), then she started changing my diet, decided I should cook at home and not eat out so much, and finally insisted I put the xbox away.

-Well, when she came back from a visit from her family, she suggested I ask my dad for money to buy a 500,000RMB apartment. I knew our relationship was doomed.

laowai spend money on beer and food no care family or gilrfriend

they say its cultural differences, they prefer their Chinese guys more than foreigners.

I thinks its because they ask too much and we give too little or we cannot agree with their norms so disagreements and misunderstanding repeated in a relationship constantly cannot work and eventually break out.

they say its cultural differences, they prefer their Chinese guys more than foreigners.

I thinks its because they ask too much and we give too little or we cannot agree with their norms so disagreements and misunderstanding repeated in a relationship constantly cannot work and eventually break out.

Probably the dating culture are different.

  Well from what I can see a lot of Chinese/foreigner relationships start online, including courtship, and the time spent to actually discover whether  they are compatible is reduced to MSN time and much smiley facing. Hardly the grounding for a meaningful coupling of souls. We all need to get laid though, right?

I agree with beaufortninja, and besides being childish also a lack of communication. Many girls here doesn't have any hobbies. Most are on sleeping, shopping, watching dvd (or karaoke). What do you talk with such a girl?

Dating a Jekyll and Hyde girl wont help with mood swings.  When The Chinese partner does not realize you will not tolerate there BS.  Western's date for love and not as a business arrangement.  This is not the case for everyone though as my Chinese girlfriend is "normal"  maybe been abroad to study and have her eyes open to the world.

When you find out her sister is better in bed.

The language barrier and cultural differences seem to be the biggest problems.