Market forces, etc. Once the set is shipped the maker's programmers usually are set to work on the next year product, fixing bugs but the sets have shipped and sold. Your car doesn't get updates either. Remember this is not a PC where we have an easy path to get more apps.

The path to getting apps on a Smart TV have a gauntlet to punch through. Here's the picture I took from a seminar on that.
http://i.imgur.com/GVlYYvo.png

As to the dropping of apps, as those authors find they can't support the older models, they have to decide what to do. Make apps for new models with all the features or ?