It's very hard for people to get in the top10/top20 if they start playing now. Even if you're really good and get a high kdr, it won't be enough. You have to play countless hours to get in there, no matter how good you are. If the ranking gets reset, people will start at an equal rank, making it more fair.

I don't get the fuss about rank anyway, it's so trivial, and far more an indication of playtime than skill.