Krakoa allows the mutants to colonize on it bc in exchange they give off their psychic energy for him to feed on. Why would he allow a bunch of non-mutants to reside on it? They have nothing to offer Krakoa and are parasites that use up its resources and provide no way of replenishing it. Shogo is an exception and they might make more exceptions but it makes sense that most humans are not allowed to stay on the island long term