Why is it a requirement to have made at least 50
posts?
As a reference, I quote
SMF Challenge General Rules
1. To enter you must have been a SMF member for at least one month and have a minimum of 50 posts at the time the Challenge is posted (sorry but no exceptions on this). Supporting Members can enter immediately, the time and post requirements are removed.
There are rules for SMF forum members to follow for participation in several areas of the forums, including participation in the official Challenges. The 50 post minimum limit is a rule for participation in certain sub-forums as well, in addition to length of membership on the forum itself. So participation in the Challenges, is less restrictive in it's prerequisites than some other activities. For example, to participate in the General Business Forum here at SMF, not only do you need a minimum of 50 posts, but also you would have to have been a member for a minimum of 3 months. And to post in the classifieds, you must be an Active member (as opposed to someone who signs up and never actively participates) in addition to a membership length and post-count requirement.
The admins developed these rules over time, which I assume was to ensure that members who posted in these forums demonstrated a vested interest in the community and the areas of interest, as well as have been around long enough to learn how the community interacts as a whole. I am not an admin now, nor have I ever been here at SMF, so am not fully aware of all aspects of the whys and wherefores and thoughts expressed among each of the admins at the time, but I have seen folks come and go in forums without regard to how the community as a whole works, and sometimes disruptions occur when that happens. That's basically why forums have forum rules and guidelines for conduct and participation.
As for why 50 posts, I'd say it's to ensure (hopefully) that the person who wants to participate in the Challenges also participates in the forum as a whole and doesn't just join one day, lurk for a month, enter a challenge, and disappear, never to be heard from again. Incidentally, the 50 post minimum is waived as a perk for Forum Supporters (recently added.)
Oh, and just to clarify, the 50 posts rule for the challenges includes 50 posts as of the date/time the Challenge was posted, so if it was posted on the 1st of the month and you had only 40 posts at that time, you won't qualify for that month's challenge.