I think that's actually a very interesting question. It's certainly possible that the accelerators here encourage exits instead of world domination, because it's 'safer' to exit if you have the opportunity (i.e. why soldier on for an IPO when you can be acquired, the accelerator makes its money back, you make some money, and everyone in the region can say 'oh well startup x was sold, look at the success!'). Continuing on represents risk...the company may not IPO, or may not continue to grow at a rate that supports the valuation, etc.
I've not gone through an accelerator here in Pittsburgh (or anywhere else for that matter), but I would strongly suspect a large part of it is the cultural mindset here--an exit is a win, because compared to their peers, that's enough, just like how our high school football teams view winning the local championship as more important than winning states. Not saying that an exit isn't a win (it certainly is), but you don't see anyone around here swinging for the fences and taking that continued risk. My $.02.