I suppose theoretically but I struggle to think of any diversionary airport that couldn't handle a 747 and also has easy access to emergency medical facilities when flying from Washington to Zurich. Lots of big airports on either side of the Atlantic and even the mid-Atlantic islands like the Azores or Iceland have runways that could take a 747. I guess maybe when you're alongside Canada and before it's closer to the European side you might be able to get a 757 into St Johns in Newfoundland but not a heavy 747. But Halifax isn't all that far away (for an aircraft) and that could certainly take a 747.Would a 757, being smaller, have a wider choice of diversion runways in the event of an onboard emergency?
Reckon that the simplest explanation is the best. The booked aircraft suffered a technical issue and the other 747 was down for maintenance