I believe that more than sales considerations, which of course are a factor, there's also the fact that Maiden like to do things a certain way, they always had and they always will. It's basically 20 years that they tour in the summer with the odd exception, and their years are planned in advance with no intention or desire for a variation.
With other bands, you know never know when they'll tour next - they might have a spring tour or an autumn tour, they might do a random tour, they could do a coheadliner tour.... Iron Maiden by now like to tour just in the spring / summer and the only variation you get is which continent starts first, or if they use a plane or not. Might be a lifestyle thing as well, maybe the 6 band members are so used by now to work with the band in the summer that they don't even imagine piutting that aside for a October - December indoors tour or whatever. They wanted to dedicate summer 2020 to LotB and summer 2021 to the new album tour, and they postponed it all of a year, and Eddie knows what Rod Smallwood is planning right now because I'm afraid concerts in summer 2021 are too uncertain by now.