This issue has been coming up for many years - it previous discussions the explanation was that each regional carrier has its own FAA Operating Certification and the rules about carry-on bags are part of that. As I had it explained to me, the PSA operating certificate does not allow any roll-aboards in the aircraft cabin.
Thus it is not an arbitrary rule, or a power-hungry FA, but a management choice that probably reaps benefits elsewhere (to compensate for passenger irritation).