Because they can and it looks as good as 3 today. They finally got the mpeg 4 compression to pay off big time. This means more room for more hd channels and the picture quality is supposed to be as good or better than mpeg 2 or 3 hd channels in mpeg 4.
Can anyone recommend a good site (outside of wiki) to understand why MPG4 is better now than MPG2? I always heard MPG4 was worse than 2 but it sounds like they've improved it.