From what I've read the huge size of the US Army helicopter fleet allows it to reach a critical mass with regards to pilot training and retention and maintenence.
Anecdotally I hear that the British Army is struggling with Apache availability.
As for us it is arguable that the then new Blackhawks and old Huey gunships are battlefield assets and therefore work be with the Army alongside the Kiowa. But now the Army is looking at 7 Chinooks, 40 MRH-90s and 22 Tigers all of which are expensive and capable pieces of kit, far more so than the machines that were handed to the Army in the late 80s and the Army struggled to assimilate.