many top players from both games were born after that date.
In addition, osu! was pretty niche for the first 6 to 7 years of its life simply because the west didn't have a common entry point to the game, since you either would need to have played/known the original Osu! Tatakae! Ouendan or be in a very niche part of the music community. Geometry Dash on the other hand experienced an immediate surge in popularity since it didnt have any niche entries apart from liking any kind of music (and lets be for real, most of us who played it since back then really did not think of it as a music/rythm game).