I guess my wife and I are just mundane - I carry a 17, she carries a 26. I guess that gives me an excuse to "fill in the gap" with a 19, huh?
Part of the reason we went with 9mm was ammo cost - we could buy Blazer 115gr. FMJ's for $3.86 per 50 before the recent price hikes. We also wanted to feed the same ammo to "simplify" our lives and ammo inventory.
Other than those two reasons - there really wasn't a good reason, other than the most important - they felt "right" to each of us when we were renting guns and trying out different ones prior to purchase. We just shot them well, and they "felt right".