Power metal is awesome, it is also, as some pointed out, cheesy as hell. I mean, I dig it...to a degree. But some of it I just can't take seriously. Helloween is probably the first power metal band I latched on to, and I dug the humor in the first two Keeper releases, particularly part 2. But then that became the template for European power metal, and a lot of the bands that built off of that into the 90s, 00s, and today really just went overboard.
But for the U.S. specifically, these are likely the reasons power metal hasn't really caught on with the mainstream:
1. U.S. infatuation with pop stars/reality shows/drama
2. The sheer size of the U.S. which makes it almost impossible for niche acts to really gain popularity
3. A stigma against metal that continues to exist
>>>>For smaller countries, those really aren't obstacles. But here, it certainly is.
All that said, however, in little pockets, you see some of these power metal bands filling up venues. Sabaton opened...I think for Iced Earth four years ago. I never would have seen them had it not been for Iced Earth, and to be honest, a huge contingent of the crowd was there for Sabaton.
So these power metal bands do have a cult following. But the U.S. is just too difficult a market for most to crack these days.