Well, remember that the maps made here are entirely user-created, so everything that's made is because someone wanted to make a playable map of a song they like. And that's kind of the nature of things. osu! primarily has lots of anime songs or rhythm game songs due to the fact they're popular, easy to time, and the general audience of players like these genres of music more so than others. There was a more fair ratio in the past, but with the direction mapping has oriented itself towards, many older songs would be found as too easy, generally, or don't mesh well with the modern style.