Too much time spent switching from one mode of locomotive to another, separate shop facilities, maintenance of overhead wire & distribution systems, raising the roof( or lowering the floor ) for wire clearance, diesels can go anywhere. There's several reasons why.
It's unfortunate that U.S. railroads, being investor owned, cannot overcome the HUGE costs that come with electrification, namely the initial cost of erecting the wire & distribution systems. Locomotive costs could be another issue, as no manufacturer has built large capacity electric freight locomotives in 40 + years. Of course electrics last twice as long as diesels, so the costs would be spread out better.