I mean, I think the why is answered in [1]. I think a lot of people like being in control of what goes on their network. For those who care about personal network security I imagine there's a big desire to not have a blob of closed source code sitting on (or anywhere near) their home network, or for that matter, their corporate network.
Also smart features on most TVs feel like they are fairly slow and seem to get in the way of using the TV as a device driven by a media stick etc. If I've decided that I only want to use an Apple TV (or Roku, or FireTV or my L33z plex client running on a Raspberry Pi), then I don't want to have to wait for my TV to load and boot its own network stack and pile of applications.