Its just the same T Rex that you saw in the first film. Whats so hard to understand or accept about that. I have no idea what the lifespan of a T Rex was but I would suspect its at least 50 years so this film being 20 or so years after the first film, it makes sense that its the same T Rex. Whether its been recaptured and out back in the park or its now a wild T Rex that lives on the island and not part of the park we'll just have to wait and see. We dont need a plot reason, it just is the same one.