Maybe because someone needing a fact on the mainframe realizes that almost no one on StackOverflow will help them, plus many will make fun of them for being a dinosaur?
Edit: I should add that I'd bet they DO have their own web forums, and a lot of them are probably run by IBM itself. (I'm not a mainframer, in case you're wondering)
These forums are even worse than Stack Overflow in terms of newbie friendliness. 'Ask your systems programmer' seems to be the reply du jour just as SO's default is 'closed as a dupe'.
Reading Red Books has better ROI than trawling for help on the 'Net.
Everybody is welcome to watch Moshix channel on YouTube, though. It's refreshing.
Edit: I should add that I'd bet they DO have their own web forums, and a lot of them are probably run by IBM itself. (I'm not a mainframer, in case you're wondering)