Marketing, plain and simple. Don't be fooled into thinking that these companies are giving you what you want when you want it, rather they are carefully calculating market strategies with the assumption that many people out there are always going to want the latest and greatest there is. They are in business to make money - we as consumers are playing the game because we want the nice toys. Personally I think the 21:9 TV's may end up being a niche market - they're great for people who only primarily watch movies say on blu-ray (and most DVD's) where the original film's aspect ratio is pretty much left intact. This obviously eliminates the black bars at the top and bottom of the screen, but one has to also consider regular television programming, with respect to the numerous HD channels out now, are all framed for optimal 16:9, so on this TV you're going to end up with black bars on either side while watching regular HD programming. The cool thing about this though, and I think that Vizio had the foresight going into this, is the ability to stack widgets and other useful icons on either side in the letterbox area, so at least that screen space is being put to some kind of use, rather than being wasted.
Either way, I'm interested to see both the new Panny VT-25's and the 21:9 Vizio's when they become available. HDTV technology is definitely taking some interesting turns these days.