If there is a reason other than not being available I don't think it is a matter of being not good enough more so just specialised in a different area I'd say. Playing live and playing in the studio are just different things, maybe John does not even want to do this on like 15 songs, who knows. Some people apparently cringe at the thought of being a session/studio musician. Even Meat has said on numerous occasions he hates working in the studio, obviously he can't be swapped! Interestingly Steinman never wanted to use the band members on the albums and this was one of the reasons that caused a rift between Meat and Jim in the late 70s/early 80s.
Also other band members (Eddie Martinez, Tim Pierce etc etc) get swapped all the time on Meat's albums as is the case with most other artists really. Only if we are talking bands is the line up usually the same in the studio and live, that's why they are bands and not individual artists obviously.
