Because the telecom industry is garbage.
In most markets, you have two choices for internet. Usually Qwest or Comast. or, in other markets, TIme Warner or AT&T.
Because of this lack of real competition, these companies have been slow to upgrade their internet infrastructure.
According to physorg, in 2009 we were ranked 28th in the world for internet speed. http://www.physorg.com/news170447728.html
I'm guessing that the company that does wireless internet on planes has a pretty advanced infrastructure in place since beaming data to/from a plane that's 30,000 feet in the air isn't easy and hence the upload/download rates are better than what you get at home.