Because "left" works fine as the verb and doesn't require "is". To me, it flows better in speech with "is", but that's a subjective opinion and probably a cult

Because "left" works fine as the verb and doesn't require "is". To me, it flows better in speech with "is", but that's a subjective opinion and probably a culturally-influenced one; British people use that construction less than Americans like me, in my experience.