They already did that in "Invasion!" with "You know, she kinda looks like my cousin." I don't see the need to repeat a joke they've already done. Humor based merely on "Hey, this actor used to play that character!" is a feeble form of wit to begin with, and it's a bad sign if you have to go back to that shallow well more than once.
No, I think it's that they're smart enough to recognize that perpetually trying to top yourself and go bigger is a recipe for diminishing returns. I've seen one or two interviews where the producers have said that they know they can't top Crisis on Earth-X and are choosing instead to go deeper in the crossovers, more focused on character and substantive storytelling, rather than just trying to get bigger in size and spectacle all the time.