According to The Register
The US Air Force, in charge of the commander-in-chief's longhaul transport, has issued a request for information under the banner "Presidential Aircraft Recapitalisation". As the airmen note, the existing VC-25 planes are modified Boeing 747-200s, a type which has now ceased service with airlines. This has made spare parts and tech support much more expensive than formerly.
According to the USAF, this means the time has come to get some new planes:
The Air Force conducted an Analysis of Alternatives to examine if it would be more cost effective to maintain the current Air Force One, or to buy a new aircraft. Given the diminishing parts supplier base, increasing maintenance time, and system upgrades that would be necessary to meet future air traffic control requirements, it was found that replacing the VC-25 was the most cost effective option.