This could again be a consequence of the amount of knowledge out there in both fields. Math is a very mature field compared to CS, though of course CS inherits some of that maturity due to how closely related the two are. First year grad students are publishing CS research because there's still a lot of "low hanging fruit" out there. Lots of open problems have the right lack of popularity, ease of solving, and importance (coupled with the fact that IME many CS grad students actually aren't that good at higher proof-based math) for this to be occurring. Over time the portion of problems that are easy to solve and important will decrease, and I bet theoretical CS's research landscape will look more like CS.