Velocity doesnt change properly

I trying to make a 2D one side collision platform and I dont wanna use effectors. The idea is to use collider.cast and trigger collider. We check what is on collider. If something is there - set a minimum velocity.y of this object to 0. The problem is that the player slowly slides through the platform at a velocity of 0.3 - 0.4. Why is this happening? Sorry for bad english.