1. It's a competitive skill based game that doesn't have teams. All the other games like CSGO, LoL, etc, force you to socialize and deal with annoying people all the time. Your rank in those games is kinda dependent on how lucky you are in matching with good teammates.
2. Osu has a way better community. You don't get told that you're bad and need to kill yourself on a routine basis like with other online games. People here seem a lot smarter and the good players aren't mean to new players.
3. Seemingly endless amounts of beatmaps for weeaboos
4. Extremely high skillcap. It's so insane how good people get at this game.