Because HERO/Champions specifically mechanically feels like a game in a way that's very perceptible to a lot of people, not like a superhero anything.
Specifically in combat it feels like a very detailed squad-level skirmish wargame, that just happens to be superhero-themed.
It got in early, and has a lot of extremely clever system design (some of it years ahead of its time), and at least some of the groups that played it in the 1980s (from accounts I've read, which were detailed and interesting, and sadly seem gone from the internet now) were very much not playing it as a skirmish wargame, but focusing on RPing the social and so on aspects of being superheroes, which I presume mitigated this factor a lot, but it is an issue.
Like, it's fine to have a gamist game, I've enjoyed a lot of them - 4E D&D for example (5E is pretty gamist too). The trouble is when it gets in the way, and the complex turn structure, detailed hex-based movement and range, precision (and quite difficult for a lot of people) character building of HERO/Champions, lack of in-built stunt/boost-type stuff, lack of support indeed for a lot of supers combat tropes (at least in 1980s and 1990s versions of HERO, maybe it changed?) but strong support for playing tactically made it quite... distinctive.
There's a reason why most supers games after that tend to lean increasingly focused on genre emulation (and later on, increasingly by being "narrative"). Because that was what people were finding they wanted after playing HERO/Champions (which isn't to say it's a bad game, it just doesn't strongly support the genre/vibes it's theoretically about - a very common issue in 1970s through 1990s games and even not uncommon with 2000s games, albeit a rare one now).