The explanation would be fog in Aberdeen, and the unavailability or unsuitability of closer alternates. It often happens that airports are unavailable for divert for specific reasons. Heathrow was unavailable to be used as a planned alternate when they changed the ATC systems in April, Stansted is unavailable during certain hours in the evening, Bournemouth is unavailable after 10pm during the Winter. Of course in a low fuel situation, it is a different matter, but with sufficient fuel the crew would've been directed towards Newcastle by the flightplan and/or operations, and they wouldn't necessarily be told the reason why.
Yes EDI is a 737 crew base, but the chance of finding a crew to come in on their days off to position the aircraft up to ABZ is far greater in a three aircraft base than a one aircraft base.