Indeed. How many want what he offers? Just because he offers a "more real" form of reality doesn't mean they necessarily want to shake the boat that's worked well enough so far, in the sense of them none of them dying in any of the occasional jailbreaks. I could see concerns being thrown around about him being the wrong color for the job (hello, implications), that mixing him in might muddle the final product instead of making it stronger, or the like. Given how the Godsmoot vote went and how many gods were so quick to throw in the towel, I can see quite a few not being willing to do anything to risk the current state of affairs even if it might result in a stronger prison purely out of the fear of the possibility of things going wrong again and consequent risk to their lives. I can also see more conventional anti-parvenu attitudes being in play as well: these upstart ascended gods are nice and all, but they're not really true gods, so we can't trust them to help build a whole universe to imprison a god-killing being. Besides, he's obviously not even in a proper pantheon, so he won't have the guidance of a proper god.
As well, how willing is the Dark One to cooperate: does he just want a place at the table, or does he want to be in charge? All we know is what he's told to Redcloak so far about his side of the plot, which makes it sound like the former, but it's not impossible that it's the latter.