Because dinosaurs are cooler? Let's be honest, if you have to decide between a Smilodon and a Tyrannosaurus, which would you prefer? Sure, the Smilodon is an incredible creature as well, but it's still a big lion-like creature with some giant fangs, so similar to something you would see in a zoo. The same goes for mammoths (giant woolly elephant) and terror birds (big flesh-eating ostrich) as well.
The main reason behind the creation of the Indominus Rex, the first hybrid was that after so many years people began to get "bored" of the dinosaurs. The wow! factor was gone, they became a common thing.
CLAIRE: We've been pre-booking tickets for months. The park needs a new attraction every few years in order to reinvigorate the public's interest. Kind of like the space program. Corporate felt genetic modification would up the "wow" factor.
OWEN: They're dinosaurs. "Wow" enough.
CLAIRE: Not according to our focus groups. The Indominus rex makes us relevant again.
Now imagine this if they cloned prehistoric mammals. This would happen much faster with them, meaning less visitors to the park.