Quite obvious why. The var ref is looking for something objective and more substantive before he has to revert to something entirely subjective to disallow the goal for.
we all saw it happen in real-time.
The Villa goal wasn't disallowed for the foul by Ramsey but because he was in an offside position when the initial cross came in. It would help everybody if supporters in the ground and watching at home could hear the conversations between officials for VAR calls so we're not just blindly guessing.