The main reason that DLP sets are not sold by most vendors is that the trend in marketing has been toward flat panels. The RPTV market was shrinking and the various vendors decided that it was easier to just sell LCDs and a few still sell PDPs. Additionally, most of the DLP makers never really took the products seriously, building them as cheap as possible to stay ahead of the dropping panel prices, and ended up with products that were rather unreliable. Samsung and RCA had so many problems with most of their sets with color wheel, lamp, light tunnel, and ballast failures that it gave the category a black eye in the mind of many consumers.
Currently, only Mitsubishi markets RPTV, and it is perceived to be a niche market. For the money, however, they have some pretty good value products when you consider size and image quality. Other than the pervasive capacitor problems in the first generation of their DLP sets, the Mitsubishi DLPs have been very reliable compared to the other brands with the common color wheel problems of others being virutally non-existent in their sets. They also currently sell their lamps for $99 retail, making that cost issue mostly insignificant.
Many vendors still use DLP in projectors, as it is a very effective and elegant solution to the problem of getting an image to a screen.