Because they'd lose out on those that still had standard televisions, which is still many, many, many, many people.
We used to have a 60" standard TV set. Had it in my room for awhile, amazing. Someone had got rid of it from work and my dad brought it home. If it wasn't the fact that everything looked a bit dark on it, we probably would still have it. But someone else does now, after we sold it at a garage sale (should have been here, lol, we had to put a sign up because the first guy got it, and everyone was going after it).
They're still really nice to have, and they're pretty cheap right now. My current HDTV in my room is only a 16" and cost me a little over $100. Very nice, but probably the smallest TV I've ever had, besides my 5" Sony I had for a year or so. A lot of people can't afford a 32" like we have in our living room, and obviously, it's not a priority for most.