1) Ignorance. Most people just don't now about this system.
2) The masses are fools. Most people tend to do what everybody else does, and "everybody" is playing D&D.
3) A horrible lack of support. This was the second most popular RPG at some point. And they blew it all away with the horrible lack of support they gave RuneQuest. Most brp fans blame Greg. I know I do!
SGL.