State police are the default law enforcement in PA, and as far as I know that has been the case as long as the state police have existed.

Ultimately, sheriff's deputies are the officers of the courts - not cops. Sheriff's deputies can be (but are not necessarily required to be) fully qualified law enforcement officers but routine law enforcement is outside the scope of their duties. Deputies are eligible to serve in interdepartmental law enforcement groups like regional SWAT teams.

"We're a Commonwealth" comes up as a rumored explanation for a whole bunch of things, but it's legally meaningless.

There are a few other states (all in the northeast) where sheriff's deputies have a similar or even smaller role.