[jctone]

I think that's a part of it. But there are many reasons. I'm apart of the Collectors Universe Forums (PSA forum) and probably 99.9% of the people there prefer PSA, and keep an eye out for how many people on the BGS forum prefer PSA, I've seen more than I would like to.
Many think PSA has stricter grading standards (which they don't, Nolan5000 is right, BGS has stricter centering standards, just compare both of the grading standards on their websites).
PSA is a publicly traded company which means you or me can invest and buy PSA stocks. And from what I've seen, PSA cards typically, (not always) get more value depending on certain factors, take for instance the PSA 10 Fleer Jordan rookie vs. the BGS 9.5 Fleer Jordan rookie. On top of that, PSA is accredited by the BBB (Better Business Bureau) BGS is not.
Also, the PSA set registry is like a deadly drug to collectors. People will pay BIG MONEY for a pop 1 or 2 of an average player to complete their registry. BGS' registry isn't quite that competitive, it's also a lot newer though.
But like I've said in a previous post, there are good things and bad things about both companies and it's really not worth arguing about.

I enjoy your take on the two. I wish BGS take more action in promoting BGS services to the general public that is not aware of BGS. They should be promoting their strong points, like higher standard grading and improve the registry to make it competitive. They could achieve this like allowing special discount for people who grade whole collections that boost up certain targeted population counts within BGS.