One big reason is the division in the community. There are now two PG tournament sites using the API. The number people playing tournaments haven't gone down as drastic as some people make it out to be. The numbers have just been divided is all. There's only so much time to play.
Do you mean the ladder site?
Player base simply is too small. Without a big marketing push this probably won't change in the future. That only 10% play a game they purchsed regularly isn't that uncommon. A look at steamspy shows that sells and regular players aren't nearly the same.
P.S.: I don't want to take anything away from anyone, but that we have two main tours right now (PGA and European) isn't that helpful for bigger fields as neither.