Well, one reason, and its probably too flimsy to satisfy, is I have been trying to get a fairly set day for FriKOTH, usually the 2nd Friday of the month. I was hoping a set day/time might help attendance as people would 'know' when the event was. This is becoming problematic on the scheduling side recently though.
I can usually set aside a late Friday afternoon/evening pretty regularly but midweek would be much more difficult for me.