I don’t think it’s that complicated. There are clearly tiers of characters; ones who are big enough that they warrant evergreen waves (Spider-Man, the X-Men, Iron Man) and ones who only really get one when they have a movie out (Thor, Doctor Strange, Black Panther, the Guardians, Captain Marvel, Ant-Man, etc). Thor is pretty solidly in the latter, so that means most of the time we get his characters are in stuff like Avengers waves, where his cast is competing against literally every other C and D list character in the Avengers franchise that could also go in those spots.