Presumably because any increase in speed, whether inadvertent or not, will bring you closer to Vx. Therefore should you inadvertently find yourself, engine out (or not) and above your V2 speed, you will then have a better gradient than at V2. True, you would have compromised it by allowing the speed to increase in the first place, but why then bring speed back to V2, and therefore a higher drag speed.
I think.
All the best