Ultimately, part of it is simply logistics. The more people, the more effort it takes to keep the contest results current. However, the main issue that has occurred in the past is that folks would come out of the woodwork only at signup time never to be heard from again. As you pointed out, it's about building camaraderie so there's some benefit to adding guys who are active here all year around or even at least during season.
That said, I'm not sure what
@redsquirrel is considering this year. This is the first year that we've had paying memberships as a pre-requisite so all we all can do is await and see how it unfolds.