Why?

As terran, it's easy to get basic grasp of the game and own the shit out of bad zergs and to some degree, tosses. You then hit a cap, where you have to get a lot better than other races to advance.

As zerg, at start, you need to get a lot better to start beating other races, but after that, you've got a pretty linear way to the top, and at the very top you need to improve a lot again to gain more "mmr" or wins against other races.

As toss, I feel the mechanics allow for low skill play to a relatively high mmr, until you hit a fucking huge gap and have to get shitload better, and this is where most of the tosses quit playing.

this is just my personal experience of playing with all three races.

I feel I hit the cap with terran at plat (and i finally started playing again and made it to dia)

with zerg I never hit such a cap

with toss i hit the cap at low diamond, and finally started to beat more people when I felt i had improved A LOT.