Why is continental crust less dense than oceanic?
Thickness has nothing to do with density. The density is how much a given volume weighs. If a block of 1m × 1m × 1m weighs 60kg, and another block the same size weighs 100kg, then the second block is more dense than the other.

So if the material that makes up the continental crust is less dense, then the continental crust will be less dense.

This almost sounds like a homework question, so I will leave references and such for now, but finding densities and compositions of these varying crusts is not too difficult.