carbene intermediate stability ........

FIRST OF ALL :CF₂ AND :CCl₂ are stable than CH2 as resonance effect . on the other hand f is more electronegative than chlorine. and chlorine has large size.

Chances of Backbonding seems 2b a reasonable reason. [4]

cf2, ccl2 and ch2.......

Because of the large size difference between chlorine and carbon, the 2p and 3d orbitals are of greatly different energies. Thus resonance is less prominent than inductive
carbene is an electron deficient species so -I effect will destabilise it...