Is it really that much load on Wikimedia sysops to install a (very simple) script like OSQA? For the value added it would pay itself quickly off. But this goes down to resource allocations and innovation potential at the Foundation, which I can not understand most of the time...
It's simpler than that in my mind... other people do Q&A much better, why bother with the hassle :)
That's just my 2p; I think self hosting is a bad idea from both a community and business perspective, but I won't belabour the point :)
Tom