I think it has to do with Judas Priest having actually been a popular
mainstream band in the US (the upward curve 82-87, culminating with the Turbo US tour and including a participation to Philadelphia's Live Aid in 1985). The people who went and saw them in concert in those days were potentially the same audience as that of post-
Permanent Vacation Aerosmith,
Hysteria-period Def Leppard, Van Halen, Scorpions, Motley Crue, Ozzy, 1987-1991 Whistenake and Guns N Roses.
Iron Maiden - I think - stopped short of achieving this status, even in 1984-1985, in terms of "mainstreamness".
In other words, JP had its American period (which I can't say Maiden ever really had - only as a nice accident, even though it has been said that "Flight of Icarus" was a deliberate attempt to seduce the American market) and possibly now feel entitled to being recognized as an RNR inductee - which would make sense regarding what they did in the 80s. Despite the fact I consider JP as a defining metal band, I tend to associate their act to something much lighter than IM and Black Sabbath, in terms of "true metalness" if that ever makes sense.