Il2cpp on average has better performance. That's not the main reason though. Main reason is making things consistent across all the platforms.
WebGL and android require il2cpp.
Windows and linux can have both mono and il2cpp.
Now that I use il2cpp on windows just by testing the windows version, I know there won't be any unexpected bugs related to il2cpp on webgl and android versions of the game.
Let's not talk about mac that won't let me build il2cpp unless I buy a mac and requires me to use mono for it, but I realized only after I finished work on making il2cpp work on windows and linux.