Laziness, money, convenience
I have to agree with the fact that a large factor is laziness and the built in apps not doing much that people want.
For example, I know someone who bought a pricey Sony tv with all these bells and whistles and it's been over a year and it's still not connected to their wi-fi. They do pay for Netflix but they say setting up streaming is too much hassle and they prefer to just get the DVDs as what they usually want is on DVD, not necessarily streaming.
On the other hand, I have an internet connected tv, but I also have an HTPC using Windows Media Center. Most of the apps on my tv require a subscription (Hulu+, Netflix, etc) so I don't use those. I end up using my HTPC for almost everything. There's only one app I use on my tv (rarely) which is Crunchyroll so I can catch some anime (especially since Disney seems to have given up on showing an edited Naruto Shippuden). If they had a WMC plugin I'd probably use that instead. All Crunchyroll has is a Boxee plugin, but since I record some shows OTA, I use WMC instead. If they had a WMC plugin, I'd probably never use any of the internet apps on the tv.
If manufacturers want people to use the tvs online, they need to address the three factors of laziness, cost, and convenience. I doubt they'll do it though as any extra options they can toss into a tv to inflate the price makes them happier.