IMO, a few factors helped Maiden to regain their popularity since Bruce and Adrian came back:
1) Management and marketing: Rod for sure knows his job way better than Priest's management, and really took the good decisions about how to promote the band and bring them to the top again (the mix between album and history tours is a perfect example how to keep fan's interest awake).
2) Live performances: Judas Priest and specially Rob really do look their age. I saw them in 2015, I was in the front row and it was really painful seeing Rob performing, not talking about the vocals but just his presence on stage, I couldn't help but thinking "please stop, you're hurting yourself old man". The rest of the band, apart from Richie, doesn't do a lot on stage either. In this area, Maiden are on top of their game, specially Bruce and Steve look like they're still in their 30's running all over the stage and even the more quiet ones like Dave and Adrian look healthy and not ready at all for a retirement home.
3) The music released since both of their reunion: for me since Maiden is by far my favorite band, it's hard to be objective but I listened to all of their albums and I feel Maiden isn't as nostalgic as Priest (per example, on Angel of Retribution, it really feels like they tried to use the formula of their most well-known albums and songs, in order to please their fans, way more than what Maiden did with BNW).