In my opinion, Surefire is the cream of the crop for mass-produced (non-custom) flashlights. While customer service is occassionally suspect, the products themselves are generally top-rate.
I think the primary reason that there are alot of Surefires on B/S/T and other locales is quite simply that there are alot of Surefires out there. Period. There are so many Surefire products, and so many different versions of their products, that there is naturally going to be a large market and alot of activity for them. Katokichi Ishishiki, Mr. Bulk, McGizmo and other custom producers create exceptional products, but their market share is so much smaller that you simply don't see them as often as you do Surefires.
A secondary reason is that Surefire produced an incredible number of high quality incandescent lights prior to their (somewhat late) entree into LEDs. Many people are simply converting their old incan's into LED's, creating even more Surefire sales.