Why will the Tories (probably) win convincingly? Because the alternatives are unpalatable to so many people, even traditional Labour or Lib Dem supporters. It's the "better the devil you know" mentality. Neither Labour or the Lib Dems are seen as caring about people any more than the Tories do don't.
The truth is I don't think that Labour really even want to win at this point in time. They know that Corbyn would be a very unsafe leader for the UK, they know that the only thing he is really capable of doing is keeping them out of office for another few years. In five years, whether Brexit is a disaster or not they will be able to claim that the Tories have fucked it up. They will be able to elect a new party leader, someone more in the "New Labour" mould, and sweep the boards in 2022 in much the same manner as they did in 1997.
Labour don't want to win at present. Brexit will most likely be toxic for every politician and party involved in it. It's much easier to stand at the sidelines and criticise than it is to make Brexit work.
If Labour seriously wanted to win power, they would make a commitment to give the UK a referendum on the final Brexit deal. A referendum to take the deal, leave without a deal or to remain in the EU. The Lib Dems, Greens and Scottish Nationalist/Socialist Parties would probably be willing to do a deal in marginal constituencies so that the "remain" voters weren't diluted.
I'm not saying that this is what I think necessarily should happen, but I think it is the only realistic option for Labour to be elected in June. Which I don't think they really want.