It's because the prehistoric thing took a little while to come into effect. At first all the Creatu had birthdays of the day v2 started, but then after a few hours or so all the ones on people's profiles were changed to prehistoric.
So everyone who had put their pets back in their Showroom already kinda missed out. x3
... Or at least that's how I've always been pretty sure it was. I remember seeing some people say they didn't get prehistoric pets, and all of them had put their pets away, so.
And if I recall, my pets weren't labeled prehistoric in v2's first few hours but then after a little time they were. All of them were still on my profile at that point.