Yeah, and again I think fleshing out the side characters is often perceived as something you do when you've got a good thing going with the main character. But if the perception is that Superman is broken then you end up contorting him in ways so that he breaks his status quo, which means he's divested of that supporting cast. Off handidly, you have Grounded, you have Truth (which did some great work with Lana...Pak was not allowed to use Lois), you had Lobdell constantly trying some new thing, you had the Wonder Woman thing playing out across books. Before that you had New Krypton.
It's standard practice to disrupt a status quo, try something new to get your character out of a rut. Eventually you steady yourself and establish a new status quo and you build your characters as you build your status quo. But no one ever gets to that point, because sales decline, fans get angry, the quality often isn't there and so everyone sulks off to the corner. You've wasted a year where you did nothing with the supporting cast you had, anything you set up is scuttled and so anything you built is forgot (and, again, often it deserves to be)....
Look, part of it is editorial, and it has to be. There was talk of Landis doing a Death of Sueprman thing for the main continuity and it got scuttled. And then he goes and he does a really excellent, maybe best post-All Star Superman run on Superman, out of continuity. So there has to be some element of the company, and likely at the editorial level, coming in and stopping people from doing good, daring work with the character, and forcing directions and deformations on the people doing work on the titles.
But part of it is, I do think it's hard for people to get on board with Superman in general. It's harder to make that interesting; a lot of what works is subtle and easy to neglect.