I think mostly, people want something official and up to date for their favorite setting. Sure, we could convert Dark Sun to 5e, but so much of that would be doing more than some people are willing (or able) to do. So, let's look at a more traditional looking D&D setting: Dragonlance. There's quite a bit in there that is different. Some races (dark elf isn't just drow, kender are different from halflings, gully dwarves, dimernesti/dargonesti elves with their shapechanging, irda), a few of the classes, or at least subclasses (the 3 wizards, solamnics). Sure, a DM could do the conversions themselves, but the thing with homebrew conversions is that not everyone does it the same. How many different warforged or kalashtar have been made before Ride from the Last War?
WotC could make easy money publishing a Dark Sun, Planescape, Dragonlance, etc setting book, including all the things that are unique to that setting, and then forgetting about it until the inevitable 6th edition and the whole thing gets regurgitated. Maybe throw in a bit of lore since the last time something for the setting was published. Or leave that part completely alone.
I still don't understand WotC's aversion to lower page count, B&W soft cover books (or even full color) since 3.5 first became a thing. Something like that would be perfect for these things.