I think people are wildly overstating the impact static setlists would have on the general public. The hardcore fan base is NOT a majority of concert attendees. Heck, I'd wager they're not even in the 25% of the audience at a concert. My experience is that most people who are attending ANY rock / metal concert do it because they're a fan of music, and like the band, not because they're hardcore fans of the band they're seeing. And -that- audience do not care whether the band played the same songs in a different city they went to the night before, because -that- audience didn't see -that- show.
Of course Setlist Scotty has some issues with static setlists, as it's made his "job" redundant and boring
DT has been touring a lot more intensely in certain areas the past 5 years. I've seen them 12 times the past 5 years without even having to travel outside for Scandinavia, and before that I saw them every other year, which amounts to 8 times since 1998.
I'd say it's a combination of high activity in a increasingly saturated marked, not only from DT itself, but every other active band on the planet. I know for myself that I've had to weed out the interesting bands to a much higher degree than before, and simply not attend concerts I previously would've not hesitated going to, just because there's just too much! There's also many more bands out there now than there ever was before, and that doesn't help with the saturation.
And, of course, DT never reached the level of "legendary" that bands like Metallica and Iron Maiden are, who can sell out stadiums to people just due to their name. And DT isn't old enough to fully reach that nostalgia level of impact where people will turn up "just to catch a glimpse of the artist before they are gone", like you get with Yes or other 70s prog/rock acts that are still touring.
But has DT's popularity decreased? Of course, that has also been a factor. DT used to have a big portion of uptake of younger generations entering the fanbase, but this kinda stopped around 2007 or so. It's the same "problem" that the guitar makers are seeing - younger generations are simply not as interested in rock music, so the guitar is actually quite struggling with dwindling sales. So when young people no longer get turned on to DT, and the older people get increasingly disinterested, we end up with decreasing popularity. But at the same time, we (Dream Theater World) has seen more activity for the fan club than I ever saw with the old style fan clubs...