Because "flat" is more in than "big". A lot of people shop for aesthetic reasons. DLPs, no offense because I love them, just aren't really aesthetically pleasing...
You might be thinking, "Well, I'm a guy! I love BIG screens!" but your guy thinking is often overruled by the wife or girlfriend factor. They want the cute, sharp TV they can hang on the wall. The thinner, the better.
Sometimes, though, it's a matter of brightest, inaccurate, most colorful picture too.
DLP just can't win in this unfair match up, so they don't sell. Thus, the stores remove them from the floors.