Because the NDP, Liberals and Greens all basically collapsed.
Trudeau hate tainting the Liberal Party, there was little chance of Crombie winning, and her tending to lean to the right didn't help at all. Unfortunately, Marit Styles looks similar enough to Crombie that she could be her younger sister; which no doubt didn't help the NDP in any way (I personally saw people mistake Marit Styles for Bonnie Crombie on a number of occasions), and Mike Schrier has a good message, but not enough oomph to spread it.
I think the polls will change if we see someone with a strong progressive message and the ability to get it out there, provided Ford doesn't play dirty tricks again by trying to limit advertising, etc.