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.
I wrote "early aughts Dem disease" but it's also the midterm election Dem disease that we still have with us. In 2010 and 2014 I couldn't have told you what the Dems were running for. Yes all the smart political consultants tell us these elections are local and it's wrong to nationalize them and blah blah blah. They collect a nice paycheck. And lose. Huzzah.