Q:

Conservation of kinetic energy in collision

Why is kinetic energy conserved in collision between glass balls while it is not conserved in collision between a ball and floor?

A:

The difference is only in the properties of the material of a body. You can see in this video

If it is elastic (happy ball) it can deform itself (thus absorbing KE) and then recover the original shape, giving back roughly the same amount of KE, which is considered as temporarily stored in the lattices

If it is not elastic the body will stay deformed and the energy spent to deform it will never be recoverd. Another reason lays in the more difficult concept of hysteresis, which is  explained here for an elastic band. In the video you see that the sad ball is not visibly deformed because hysteresis is great work done is dissipated in heat

