It is no real surprise that a Boeing airliner has been chosen. The two key parts of the initial requirement that the aircraft have four engines and be assembled in the USA. Add to this a (I understand) requirement that the design offered have a pre-proven safe service history and the field is very limited.
The first part brings the field down to three types: Airbus A340, Boeing 747 & Airbus A380.
The second requirement eliminates the two Airbus types. It is simply impractical to set up a production facility several time zones away from the existing production facilities just to produce two, or at most three aircraft (see earlier posts).
What is really surprising to me, is that the US put this out to a pointless form of competitive tendering, rather than just take the "National Security - driven requirement" and just specify a 747, end of story.
But again, I don't follow the logic of needing 'four engines, more maintenance, more fuel burn & statistically a greater chance that an engine will drop out, due to the fact that there are more of them. The routes and locations flown by AF1 are well within ETOPS, even over nominally unfriendly territory.
The sceptic in me wonders if there is something missing in the [Boeing] twin airframes that is there in the 747 that drives the four engine requirement. The Airbus designs being irrelevant here. My suspicion is that it may be something related to the air to air refuelling capability. Perhaps the fuel management systems are older & simpler on the 747 & hence more robust for upset conditions than more evolved & sensitive systems in the twins. Pure speculation on my part.