Maiden could capitalize very effectively the return of Bruce and Adrian since 1999 whereas Priest just couldn't do the same with the return of Halford and it was in part because Maiden did a great job showing themselves as a modern band and attracting new audiences.
However I don't think that Maiden's success over Judas is only explained for having a better manager. I think Iron Maiden overall have had a more consistent discography and that their output since the 2000's has been very superior to Priest's thus helping them gain more fans.
About classic albums I think both bands have more or less the same amount of classic albums but I thinks it helps that Maiden classic era was bassically one album after another (the first 7 ones) whereas Priest's classic albums are a bit more spaced with some weak albums in the middle.
I think that's a very good point. Priest have a ton of classic albums during their heyday, but each classic album is surrounded by at least one weaker album. Priest's late 70s output was actually the strongest run in their discography (that I've heard so far, anyway).
Sad Wings through
British Steel is a great run of very good albums, but none of those albums are perfect and all of them contain at least one very poor song.
The 80s/90s were up and down every step of the way, allowing classic fans to drop off and move on to other bands. The new fans the band gained along the way via the commercial route were probably turned off by the less-commercial, far more metallic stuff.
British Steel - great (earned new fans and still satisfied old fans)
Point of Entry - boring (probably lost old fans here who were already conflicted about the commercial approach of
British Steel)
Screaming - great (but far more metal than the last two albums, probably alienating some mainstream fans)
Defenders - pretty good (more metal, keeping the core fans happy)
Turbo - WTF? (lost old fans here, probably gained more mainstream fans)
Ram It Down - a middling attempt to recapture the metal sound (lost any new fans from Turbo and probably lost more old fans here)
Painkiller - amazing (but all those
Point of Entry/Turbo fans are left in the dust)
Ripper era - lost 50% of their audience due to a long period of inactivity, an underwhelming album with a new singer and a shift in sound
By comparison, Maiden had a couple of early albums that grew their sound and then put out 5 classic albums in the same genre in 6 years. When they declined, they declined once, and then boosted themselves up later with the reunion and remained active in satisfying old fans with quality new material.