I assume it is because if you reach Vmc and the aeroplane starts to depart and then you stall at the same time then it could be more likely to lead to a spin? (possibly because now you have two things to deal with...I dunno, rudder inputs could exaserbate the problem and the Vmc corrective rudder during a stall could put you into a spin?). Above stall Vmc will happen and you deal with that, below stall, you will stall before Vmc happens (and hence it won't happen) and you deal with that.
I know of one FI who spun for 6000' before recovery during a Vmc gone wrong demo, I suppose due to the wing nut effect of the engines. Luckily they had 7000' to play with.