I ‘m no expert on UK politics, though I do follow it pretty constantly, but pretty sure one problem (not the only one) is that Labour has the early aughts Dem disease. “Not quite as evil as the other guys” is not a winning strategy. It isn’t really about going more Left, though on some issues it could have been, it’s about clearly highlighting the distinctions. You know, making clear just what it is you stand for, whatever the hell that is.
Or, as Harry S Truman put it, “Given a choice between a fake Republican and a real one, the public will choose the real Republican every time.”