Speaking as a 3,1 user who decided to hold off, my reasons were:
1. I've waited long already, I can wait until the 2nd revision, assuming my 3,1 holds on for a while longer
2. Lack of Logic Pro support for Open CL
3. Still on the fence about the lack of a proper tower case & having to spend a lot to upgrade to TB enclosures, new audio interface, etc
4. non Open CL performance not that impressive compared to previous gen
I'm sure there's many who had similar reasons, and so the supply problem could be a lot WORSE than it is right now.
I think the main cause is that this is a new production line, new production methods. Once they get their yield up, hopefully the supply constraint windows should be shorter.