Going back to McDonnell Douglas days - they liked the gearbox under the fan case for maintenance reasons. The CF6-6 and -50 were configured this way, and IIRC the JT9D-59 on DC-10-40s (JAL & NWA) were also.
But Boeing was more performance oriented, and bolting the gearbox up close to the core made the fan cowl a bit slimmer. Thus the CF6-80A and 80C2 have a core-mounted AGB -- as have all P&W Boeing engines.