I just watched the first movie again and noticed that they said 7 raptors were bred. Then the large female raptor killed all but 2 of the others, leaving just 3 total. At the end of the movie 2 are killed by the t-rex. This means that there is only one raptor left on the island to appear in the game (plus any baby raptors from the lab). That's probably why they needed to introduce a "new threat."