You can charge more for a double album, and a double album counts as 2x the sales in terms of gold/platinum status. Why do you think Metallica’s Hardwired was sold as a double album when it would have fit on a single CD like Load…?
People keep saying this, but I've yet to see a reputable source actually state that the RIAA counts any double album as twice the sales, rather than the old standard of double albums that are over 100 minutes long. Either way, Maiden going for a double album once again is almost certainly due to them refusing to compromise even a bit over the song lengths.
And like
@Midnight said, HTSD was a double due to sequencing issues - they wanted Lords of Summer on the album, the producer insisted it wasn't good enough and eventually got his way, but by then they'd already put in the order for two CDs per album and had packaging ready and all. Hardwired was written and thrown in after that so they could alter small things like the running order that's printed on, but the plants were ready to print two CDs per album and that wasn't something they could change.
Even though a CD technically has room for 80 minutes audio (maximum) using it all is asking for trouble. Have heard on hifi-forums some players have issues with tracking when getting above 70 minutes.
Since we're on Metallica, Load is 78:59 long, apparently the exact maximum they were given before it would start causing problems with older CD players. This was in 1996. If it was okay back then, it would've almost certainly been fine now. Senjutsu is under 82 minutes so cutting about three somewhere would've been fine.
e: And just to note, I'm fine with Maiden doing a double album by the slightest of margins, just as I was with Metallica doing one when they technically didn't need to. It's an artistic decision. I'm sure that they didn't randomly pick that the first disc has six songs instead of an even five-to-five split, for example. I'm just surprised they went with a double album at all with how small of a margin it is.