Yeah, US is mostly protestant. Still, really modern Catholics are a minority. IIRC, the majority is from less developed countries in South America. I'd think their views on faith would still be pretty conservative.
If The Church was democratic, we'd never see reforms like those just coming about. Democracies are prone to getting stuck in their current state, and The Church is already suffering from this. Also, unlike with countries, stability is not really a concern with Catholic Church. If it becomes too unstable, people can leave. As such, monarchy is one of the best ways it can be governed, as it can be much more dynamic than any democracy.