Because the form factor of a laptop doesn't suit a huge portion of the media consuming public. Many people would rather laze on their couch with a tablet in hand and have the tactile feel that touching stuff directly on screen gives you. Also a lot of people have said that using a laptop or desktop 'feels like work' and that they don't like feeling like they are in the workplace when they are at home. For browsing and consuming information, for reading ebooks and for looking at videos and pictures and sharing them with family, I much prefer the tablet aesthetics and handling, passing a laptop around is far clunkier.
When I have work to do, I go to my desk and fire up the i7-950 and the big screen, but when I'm relaxing I find the tablet consumes much more of my time than my i7 2.8 laptop, which is gathering dust most nights.