The thing is that while Canada is a Center-Left country, there's only so many left-leaning voters to go around.
As a left-leaning party, the Greens have to compete with both the Liberals, and the NDP, for votes. As a left-leaning party, the most the Greens can hope for is scrounging up proverbial crumbs of support that decide not to vote Liberal or NDP.
The old Tories, particularly under Joe Clark in the 90s, were an unique party. They were fairly centrist, and I remember reading that even David Suzuki considered running for them. The old Tories had strong support from small business farmers due to how their policies were seen as friendly to such farmers (they made a point to target such voters). The same policies that made them friendly to such farmers made them, by default, very environmentally friendly policies due to how small family farms are themselves very environmentally friendly. This is why you even had people like David Orchard - called a "Green Tory" - supporting the PCs, and even running for leadership of the party.
The old family farms vs. megacorporation farms issue served to unite the old Progressive Conservatives with many environmentalists. I think that Harris is trying to take the place of the old PCs (who did, after all, at least win some seats) by moving his party to the right.
Personally, I think that it won't work, but then the alternative wasn't very appealing for the Greens' either.