Well part of the problem comes from immigrants who come to our country and make a half assed effort to learn the language. But at least they tried, I suppose. Some immigrants don't even bother and that's the other problem.
Another source for it is due to censorship. You wouldn't think it would be the likely problem, but the rap industry has to make up words to bypass the censors and still seem edgy and offensive.
Another reason, lack of education. Since the quality of our educational standards is dropping rapidly, and since kids are becoming more defiant as the generation gap increases, more and more kids are dropping out before they graduate thinking that they can cope with the world without it. Some find a way, others spend their lives in McJobs.
The main reason, laziness. Plain and simple. People don't care to spend the time to type a full thought/word into a text and abbreviate most of what they try to say.
But that's just my opinion on why the world is slowly going to turn into the movie Idiocracy.