Just speculating but I think there would be an issue with transporting all of the people in the enclave, along with their possessions, to Catalina. The enclave is a small refuge of people surrounded by a vast horde of zombies. I don’t know the exact figures but it’s clear there the zombies outnumber the people by something like a 1000:1 ratio. The humans survive because they live inside a fortified area and the zombies don’t know how to climb over walls. But if the people attempted to travel outside the walls, they’d be overrun. The only people that go outside are groups small enough to travel in one of the handful of working vehicles and the superheroes.
The other issue is resources. The books have made it clear that the enclave is living mostly off of stored food it scavenges from the city. In the most recent book, they’re trying to set up a farming area to produce food but experiencing difficulties. Catalina isn’t going to have the kind of resources that Los Angeles does.