It has nothing to do with not being sure.
It goes back about six or seven years to a stretch when the BaptistBoard was inundated with Catholics, atheists, and liberals.
Basically what happened is that the Gospel and Baptists in general were under constant attacks from these three groups and the boards became very contentious. So much so, in fact, that it drove a lot of people away.
This led to the decision that non-Baptists would be accepted on an individual basis, pending approval, but that board would remain a board largely for fellowship among Baptists and for the purpose of edifying fellow Baptists.
If you go to the main page, you'll notice a huge library of archived posts from the early 00's. If you have time to go through them, you'll see that there were a lot of Catholics here at one point.
For the record, at the time, I was furious about this purge of non-Baptists but, having been on other message boards and seen the damage that atheists and mockers can cause to a Christian message board, I now see the wisdom in the move to restrict Baptistboard to Baptists.