- It's a fairly complex game that rewards "system mastery" but not many people want to take the time to "master" it nowadays.
- Combat is complicated, though I don;t think it's much more complicated than most 3E/4E/Pathfinder D&D games and like those it smooths out a bunch with experience
- Champions books were everywhere in the 90's during 4th edition. Most RPG'ers knew about Champions/Hero System. Then the company that published it hit some hard times and it took a few years to get the game back on track with a new publisher. I think this put it behind the curve of the 3E boom when a lot of people came to/came back to RPG's circa 2000 and it never really caught up in "mindshare". Now it's on a 6th edition and it still almost non-existent in game stores as the company again has had some rough times the last few years.
- It covers a lot of genres but it was always best known for Superheroes, and in the last 10-15 years a bunch of superhero RPG's have come along, from Mutants and Masterminds to ICONS. Not all of them have survived, but going from 3 or so popular superhero games to half a dozen or more really segmented up what was already a niche genre.
(and wow that is not my favorite cover)