Yes DDO is a niche game, it's niche because it is a combination of all the harder less casual and more complex things that you can put in a game. FPS controls... the fact that you're at a serious disadvantage if you don't side step fireballs (etc.) and use mouselook. Massive hot bar sprawl, massive numbers of single actions that require their own hot bar icon and to use efficiently you'll need to make those into keyboard shortcuts for the most often used. Then there's character customization that is deeper even than the PnP game, which itself requires reading a rather thick players handbook. There's a million ways things stack or interact and good luck if you're not a D&D person telling what's what... in fact good luck even if you are a PnP vet because DDO does a rather mediocre job of translating PnP rules and has taken many liberties on top of that. I say mediocre because I compare all D&D games to the high water mark NWN.
EVE has a different demographic than "fantasy RPG's" do. You've got the Space sim crowd, the Star Trek crowd, the Babylon 5 crowd... You've got complexity there too but this doesn't make the games alike. There's also a hell of a lot more competition in the virtual gandolf and Conan market than the Virtual Picard and Kirk market. It's a rare gamer that likes all genre's I am one, so I think I have a good perspective, but I suspect most gamers like 2 or 3 genre's and that's it.
In any case it's fair to call both a niche IMO, who cares which one has more players. DDO's marketing is terrible and it's niche has far more competition for players.