Because some programs are recorded in Mandarin and Cantonese viewers wouldn't understand or vice-versa?

Local dialects change, but the characters stay the same.

Deaf people can also watch it, when subs.....

Because of dialects, even within one province there can be hundreds of different dialects and some are extremely different from the others, writing has long been the only way for Chinese from different provinces or even cities to communicate.

Think of a European country where each province (province of Spain, province of Finland, ...) would keep its own language and produce media content exclusively in this language.

The rule that says Putonghua only on TV is not enforced at all, basically.

So the actors know what to say!

Because everyone is eating and screaming at one another.

How can you be expected to follow the hilarious high-jinx of Running Man without subtitles? Sheesh!!

So that deaf people could also appreciate the entertaining programs ?!?!

So you can still follow your favorite tv show when you are in a restaurant that has several tv showing different stuff all at very high volume. 

I agree with those above; it's the dialects.  My mother-in-law will sometimes ask my wife what was said if she is seated further from the tv.  I always thought she didn't hear clearly, but then I realized that she sometimes doesn't understand putonghua.  My wife denied this this for a few years, but now it's obvious and she can no longer "save" her mother's face.  The thing is, it's no biggie to me.  It doesn't make me think less of my m-i-l because of it. 

I've heard (and believe) that China has more spoken dialects than Europe has languages.