The singlet carbene shown is much more stable than singlet methylene. What is the main reason?

The carbene shown is chloro(methoxy)carbene. In a singlet carbene the carbon has a filled sp${}^2$ orbital and an empty p orbital. The lone pair on the alpha oxygen (and to a smaller extent on Cl) overlaps with this empty p orbital, which lowers its energy and stabilises the singlet state. The carbon is not charged, not sp${}^3$, and the methyl group is attached to oxygen, too far for useful hyperconjugation.