Less prone to bad weather. Less prone to delays. Smoother ride. Contribute less to air traffic congestion. Each flight has its own gate. Higher priority for take off / landing than RJ's in congested airports. Wider range of service levels offered.

If UA really thinks that more outsourcing to regional operators is a winning strategy instead of actually flying the routes themselves, then they might as well go out of business. This practice drives away HVFs in droves. Increasing the frequency of a service while at the same time slashing quality is not attractive to HVF's.