There are probably several reasons, but I think it largely boils down to star power.
Steve Kenson is a BIG DEAL in the superhero RPG community. His Mutants & Masterminds did several things:
1) It matched HERO System's (Champions) comprehensiveness, while beating it in terms of quickness of play.
2) It piggy-backed off the popularity of d20 while avoiding its worst features.
As a consequence, he created an industry standard and accumulated a great deal of well-deserved capital amongst fans of superhero RPGs. The M&M fan base is as loyal as the Champions/HERO System fan base (which is pretty loyal), but more current. Steve's active presence on the fan forums enhanced this loyalty and grew his own personal popularity. People grew to know and trust him, because he's always been an active and enthusiastic part of discussions that deal with his games, and because he's a pretty nice guy. When he announced that he'd be doing a rules lite game (and hinted in several interviews that he prefers rules lite games) people were immediately interested. This was instant hype, and he fueled the flames, which is very smart on his part.
Now, you have industry respect too, but it's mostly for Barbarians of Lemuria. Indeed, many people think that Supers! is BoL with superheroes. This misconception might keep some folks from trying it out, thinking that they already know how the system works and they can't imagine it working for superheroes. Some people buy it HOPING that it's BoL for superheroes, and then get disappointed when it isn't (which is too bad, because they are not really giving it a chance on its own merits).
Recently, I've been wondering if the name Supers! actually hurts the game's chances of getting noticed. I suspect that when I type Supers! in a subject line, some people think I'm talking about the genre rather than the game. I started musing on this during the recent "M&M vs. Marvel Heroic vs. Supers!" over on rpg.net. People seemed not to know that we were talking about three games, so I kept highlighting that fact. Some folks actually seemed to get interested after a few exchanges.
Finally, if you are going to compare it to ICONS, I think support is a huge factor as well. Folks who bought ICONS had instant access to several very-well-done adventures, a character generator, and a few other odds-and-ends. Support always helps.
As for the art - that's subjective. I agree with you. I like Darrel's art better. I'm NOT a big fan of the art in ICONS. That, however, is somewhat subjective. I think there are MANY superhero fans out there who prefer a more modern sensibility for their superhero games, and Supers! often comes off as being very retro - very Silver Age. Some folks just don't like that. BASH UE is interesting in these regards. It strikes an Animated Series sensibility, which actually straddles all sorts of sub-genres quite nicely (Golden, Silver, and Modern).
All that being said, I would love to see Supers! achieve greater visibility. The question, for me, isn't, "How did ICONS steal Supers! thunder?" It's, "How can Supers! steal some of ICONS' thunder, and achieve the respect it deserves?"
If you ask me, they are not equivalent systems. Supers! is far superior. Yes, that's subjective too. I guess the superhero RPG community should way in on that before a consensus is reached. The issue, I think, is that Supers! remains unknown, so people haven't weighed in on its considerable merits. I'm hoping the various upcoming projects will help it get noticed.
Sorry for the long message. Thoughts? Am I way off base here?