My .02 is that it was just a generally all around better balance of size, weight, capacity, and ergonomics, e.g. it fit more people's hands mo' betta. (And a lot of the SF guys are not particularly big, just lots of "won't quit" and stamina). As noted above, it is probably a secondary/backup weapon the vast majority of the time, so giving up a couple rounds to get a bet balanced package wasn't a big deal (and of course, you can still put a 17 round mag in it if need be). And it was in the system already. As much as they practice and shoot (so I have read), I doubt the sight radius really makes a difference to them.
I have Glock 17 too, and like it, but it is a bigger bulkier gun, plus I have short fingers. The 19 is an excellent size.
This is really true. I have a family member that's SF out of Fayetteville and I've met a bunch of his team mates. In civvy's, not many of them would impress you as being a badass. None of them know how to quit though. No matter what's thrown at them, they just keep going. The saying that Fayetteville is one place you don't want to pick a fight in a bar is true. It just may be that the skinny, nerdy guy there really does have some skills that will put the hurt on you.
My guess on the choice of the 19 over the 17 is that the 19 is a more balanced weapon that's a bit lighter.