In Paris (I think, again anecdotal and I only used UberPop on my most recent trip to Paris) UberX are "citizen drivers", but meeting semi-professional standards in both their service and their vehicle. UperPop, at least feels to the consumer, like someone with a small car clicked a button saying "yep I can do this" and voila, off they go.
It's not really that, it's the car class. UberPOP is anything compact or subcompact, sometimes limiting to 2 passengers. UberX is a 4-door sedan. UberPOP is not launched in the US - you can only start driving for Uber with a 4-door sedan or higher.