It might be a licensing issue. One of the requirements of a set top box could be that it is non-hackable, so that DRM-poisoned content can safely be played back on it.

Android is only half-free. Its Apache license allows binary-only redistribution, so the vendor can make secret changes to the platform that are not re-destributed in source code. What you get is a closed system. The content industry loves that.

Maemo OTOH does contain proprietary bits and pieces, but a lot of it is LPGL/GPL. Using these underlying parts for a platform that needs the trust of the content industry might be more risky than simply using your own non-free version of Android. (I'm not saying it's impossible... Android could just be the easier way to go.)