I would think that D3s could commit later because there are no athletic scholarships and some D3s don't care about having 50 kids on the roster in the fall.
I also think those elite blue chippers that any school would love to have can wait long, if that's their plan.
But, for the D1s and D2s, I would think you want to be committed by November of your senior year or else you are losing options.
But this is all just me guessing.