Because they like playing with little cocks ?

If you had hit search when you entered this question you would have got this result:

http://answers.echinacities.com/node/98461

Saves us all repeating ourselves.

It's because it's an affordable game that people can invest in. You can get a badminton set two racquets and whatever else they use to play the game for less than 100 rmb at some places. At most a set would cost you less than 200 rmb. So when something is affordable it becomes popular. Nobody would be playing basketball if they didn't have hoops up already built. It would cost to much money for an individual person. You can buy a basketball for less than 70 kuai at some places so that's why basketball is so popular. 

fun, inexpensive, easy, can play most anywhere, good exercise, social,,,,   prolly some more reasons too.  

just like ping pong, probably easier to find space, land is a premium for most sports in china and not everyone can play at once in limited space.

Soccer is a worldwide sport because all that is needed is a ball and a field. The field does not necessarily have to be regulation sized, just a field.

Like people have stated before, Badmitton is a cheap game. Two rackets, a shuttlecock and a net, and most of the time people do not even bother with the net, just keep the cock in the air. 

Yeah it's not rocket science, space is at a huge premium here along with money generally.

Along with ping pong it requires a minimum of space. There is pretty much zero land that isn't cultivated here.

And.... it's fun. If you're looking for something to do with the wife and only child after dinner on a nice summer evening badminton beats the tv.