Honesly, I’d like an official answer from the devs, not speculation or guesses from the community. If I’ve missed a post where they explain, I’m sorry please link it to me here so I can read it.
For context, I don’t have too much fun as the monster and getting placed into a lobby with a 3-4 person group results in me getting burned out on the monster very quickly and just exiting Evolve entirely after 1-2 games instead of 10-20. I don’t mind occasional games as the monster (1-in-5 sounds entirely acceptable), however, as I said, playing monster every game or every other game is not fun to me.
tl;dr; The “groups can’t play the monster” rule makes a solo player want to quit the game. Wants Dev’s reasoning for screwing them.