I think "most people," are "succeeding," because a lot of them are just joining already existing RPs, or they have been around n the site for a long ass time and have developed either a good reputation or a group of people they can write with/rely on (or it's both of these things). Also, I think the vast majority of people on the site are looking for group RPs, and so end up in group RPs.

Beyond that, uh, it's a matter of luck, timing, effort, etc.