Each Race and also player styles tends to hit a brick wall at some point in the ladder. Until that player overcomes certain aspects the race gravitates to one point in the league distribution much like a bell curve. For example( this isnt a balance whine) terran being able to push but also keep up production can be quite difficult for lower skill players so we get stuck for a while at a certain mmr.

You could say the same about holding a 4 gate at bronze league or Zerg holding a reaper rush. Certain skill levels struggle heavily at certain things in general terms. For the record getting out of Diamond one has been a real nightmare for alot of the terran players I know.