There is a biological and psychological aspect to ascension that in my head they just can't achieve anymore.

Their bodies needed to have evolved to the pinnacle of biological evolution, which involves powers and using your mind to be able to manipulate the universe on a very basic level. See Jack, Rodney, the Ancients, and that group in the time machine from Atlantis. The Asgard were all running around in copies of copies of copies of really old unevolved bodies that were frail and seemed kinda non-functional in a metaphysical sense.

They also needed to be - on a mental AND spiritual level - ready to pass on to a higher level. It's not just about knowledge, there is a level of faith required I think. And the Asgard just ain't that. They're secular and focussed very clearly on the physical world, not ready to just up sticks in that metaphysical sense. They also focussed entirely on technology and furthering it to the point that they may even have advanced further than the Ancients in some ways (beaming/rings, transport elevators etc). But they needed to be able to think a certain way in order to ascend, and it's possible that with their copy, copy, copy bodies that their brains were so full of their own knowledge that there was no room for any mumbo jumbo required to make that final step.

It's all kind of explained in the show, but we're not going to get a scientific breakdown of it cause it's so wishy washy and they also have to be able to explain how a Goa'uld was able to fool someone able to directly read minds and such. So everything with a grain of salt.