Answer
Since C=C bonds involve $\pi$ orbitals, rotation is restricted because the orientation of the $\pi$ orbitals are definite; thus excessive rotation will destroy or significantly warp the bond-forming orbitals, a process which is energetically unfavorable and requires additional input of energy into the system.
Work Step by Step
Since C=C bonds involve $\pi$ orbitals, rotation is restricted because the orientation of the $\pi$ orbitals are definite; thus excessive rotation will destroy or significantly warp the bond-forming orbitals, a process which is energetically unfavorable and requires additional input of energy into the system.
