That was largely because Glaxo had its corporate offices right by both of those airports and they made a huge volume commitment to support the route. I was a little surprised when the route shifted to LHR, but I suppose that was to make it more attractive to a broader group of travelers.

AA also picked up the PHL-LHR route from US at one point in the early 90s (when US was forced to divest due to its tie-up with BA) and the flight number continued to RDU. I was living in Raleigh at the time and remember it being a big deal that American was offering service to both London airports.

Before London, they also flew RDU-ORY/CDG in the very early 90s, which is briefly mentioned in the first Home Alone film. I have no idea what the business rationale for that was.