Superman *wants* to sell. Man of Steel made more money than almost all the big Marvel hits, yet was infinitely more divisive. He's one of the highest grossing superhero franchises, without the benefit of constant media exposure (which Clark's "rivals" all get in abundance). Superman, despite this long running downturn, is still stupidly popular and people seem to actively want to engage with the character.
But for a long time, WB/DC haven't been able to find the right mix to maximize the potential. And I do believe that at various points they really have tried (and at others washed their hands of the thing). And the attempts they've made have often cost them. Nearly all of Superman's Returns revenue went towards paying for previous failed attempts, right? Hell, no wonder WB is gun-shy to pull the trigger on video games and cartoons. The potential losses could hurt like hell. Batman however, they do understand, and he makes serious bank. You can't blame a business for doing what it's good at and giving its customers what they want. DC knows how to play the Bat's tune and people like the sound. WB/DC is not at fault here. That's just business.
And let's not forget the legal battles, which have likely affected how DC and WB use/approach the character.
Should Superman get the high profile names? Ideally yeah, but just look at Johns. That guy is DC's biggest name and can work magic with tons of characters. Except Superman. Look at Scott Snyder. Dude's legit namebrand. But Unchained was so boring it actively hurt to read. Ivan Reis on art? Already been there (though a return would be killer). And yeah, these runs do decently but not even Johns could pull Superman out of his sales drop in 2015. But Pak made it work and he's not A-list. Tomasi and Gleason are crushing it, and they're no Morrison or Ellis (no offense to those gentlemen intended). So big name talent isnt necessarily the answer.
Superman should totally be a bigger powerhouse within DC. But whatever their hangup is, DC can't make that happen and hasn't been able to for twenty years.
They just gotta hire me, I'll fix it up.