I think a big effect is lack of radio time. The younger people live and die by spotify or FM or satellite radio and a band like DT just doesn't get air time. Metal as a genre is less popular than it used to be, and all of the other genres seem to be merging with pop, so it's a one-size-fits-all music environment with the rest being the fringe. It's sad, but true.
The ticket prices may be out of range for younger people, but I think they're fair compared to the big name stars out there. They're cheaper than Metallica, Foo Fighters, and any of the "pop" junk out there like Taylor Swift, Justin Timberlake, etc. I think anything under $100 will sell out if there is a market. If they're not selling out its because there just isn't a million teenieboppers begging mom to go see it.
Personally I like smaller venues, so I would prefer DT in a smaller place, but I go to their concerts when they come around just to support and experience the live show. It has always been worth it.