It is a legal thing, they have to pay about $5 per television sold for the patents on the MPEG-4 (H264) and AVC-1 codecs, which are the ones used mainly for videos these days. In addition, they would need a more powerful video engine chipset, so there's no guarantee your TV is even physically capable of it.
Most devices you buy, like computers with Windows preinstalled license these patents, which is included in the cost of the computer. A retailer would have a good day if they made $10 on each TV they sell (most of it is made off accessories, warranties, etc.) So $5 is a big deal.