Legal liability reasons. Being registered in an obscure country makes legal claims against the vessel so much harder.
That’s why most cruise ships except one that I know of (Norwegian’s Pride of America) are registered not in the US.
I wonder if there are reasons related to safety inspections as well? I seem to recall reading, but I don't remember where, that any cruise ships that regularly call at any American port are required to comply with specific safety regulations and inspections - but I don't know what is required by other countries.