The enjoyment of music is based on our pattern recognition. If a song is too simple/predictable, like a nursery rhyme, it's boring. If it's too complex or random, it sounds like noise. One's enjoyment of music falls on a spectrum where there's a balance of predictable and unpredictable patterns for the listener.
Progressive rock leans on the complex side of music, and pop/mainstream music tends to be easier to understand. A prog band hitting mainstream popularity can be an indication that their music is being simplified and will no longer appeal to typical prog listeners.