I'd bet that many of the out-of-towners are former DC types who have since moved away.
Like me. My new city doesn't have anything like DCUM, and BabyCenter does nothing for me. So I'm still a regular on this site.
For me, it remains a great resource for non-DC specific parenting topics, and I still feel a connection to the DC community, though I'm no longer physically there. I also like the virtual people-watching, though it involves a trainwreck or two sometimes.
As for OP's original point, I'm not sure things have changed that much. I've been a regular for a long time, and there's always been a mix of people. Sometimes that adds value and interesting perspectives. I've definitely learned A LOT from posts by people who are very different than me (especially on the Off-Topic board where the issues have more range.) Other times the diversity on this board triggers drama and distraction -- especially when conversations touch on economic differences or age differences (20-something moms vs. 40-something moms.) But it is what it is. This board is open to all. Works for me.