That helps a lot at the start of the season but not as much as it goes on. Participation is still very high because it gives people something easier to pug than raids and allows smaller groups of friends to play whenever they want without waiting for ~15 other people.
Small content always has higher participation than big content, it's not really surprising. But m+ streams also draw far more YT views and game attention than any other mode the instant the world first race is over