Probably because the majority in the legislative is not filibuster proof, and there are always a few representatives or senators who disagree with the proposal

Probably because the majority in the legislative is not filibuster proof, and there are always a few representatives or senators who disagree with the proposal and prevent it from going to a floor vote. Recent examples of such individuals are Manchin and Sinema. Lieberman was another one who managed to get the public option removed from the ACA before it was passed.