Why can't C-C double bonds rotate?
A double bond between 2 carbon atoms means there is a $\pi$ bond with the side orbital overlaps in 1 plane, in parallel to the axial $\sigma$  bond.

The consequence is, free rotation  would need to break this $\pi$ bond, making the double bond just a single bond, what would require a strong torque and a lot of energy.

By other words, particular rotation positions around a single bond have ( almost ) the same energies. But for double  bonds, the molecule energy strongly depends on on this orientation. The $\pi$ bond can exist, only if the bystander groups on both carbon atoms   are about in the same plane. If they wete on about perpendicular planes, the  orbitals of both atoms cannot overlap to form the bond.