When someone wants to get on the Internet through a TV, he will only be able to do so when nobody wants to watch a program. Furthermore, it is doubtful whether the site that dad wants to look at is the same that would interest mom or the kids. Actively watching TV Television is a passive medium. People sit in a lazy chair and let whatever program makers have to offer come to them, without asking for it. The only choice that can be made is the channel.
Websites are build for people who are sitting behind a computer.
Fontsizes and graphics can be relatively small, so a lot of information can be shown on one screen. The larger distance between a person and a television makes it necessary to adapt sites to it. Of course this is possible, but on a PC such a site will probably look terrible and on TV it will look a lot like teletekst, and we already have that. In a TV there is no harddisk, neither is one to be found in a set-top box. The costs The Net.