The thing is, with Maiden you know what you'll get. Metallica is quite inventive with the setlist, in comparison. You know there's going to be at least 45 minutes of new music, if not more, if Maiden is on the album tour. You basically know what's going to happen on the theme/history tour. Some people will get bored, and together with the inflated ticket prices, decide now's not the time.
I did not go to History pt3 tour, because the Maiden England setlist was half-arsed, as expected, the stage show lousy, as expected, and the performance reserved, as expected. Frankly I would not know what Metallica would do on such night. There would be a chance of Shortest Straw, Frayed Ends, Disposable Heroes, etc...even if it was a Lulu-promoting tour.
Especially if your target date is down the list, and you have enough shows prior that'll end up on YT and setlist.fm...yup, after a few dates, 0 chance of Still Life, 0 chance of Infinite Dreams. And the highlight, SSOASS performance, reminded me of the moments when my old walkman's batteries started dying down or when I put a 45rpm single on the deck but the switch is on 33. Sorry, I'll pass.
Metallica does this in a smart way. There's no problem if Lars (or anyone else) fucked up something they have tons of alternatives. They actually rehearse a lot, as a full band, before the shows.
Maiden is extremely lazy in this regard.