Canadian hydroelectric is the catalyst that makes winter hothouse produce cost competitive. Wealthy us elec producers have no incentive to match Canada's low cost of production. Indeed their incentives are rather contrary.

Most of the tomatoes and cucumbers I buy here in Alberta year-round are greenhouse grown in-province. Our electric power here is basically natural gas with a bit of wind and hydro. Although to your point, we're probably one of the cheapest locales in the world for NG.

In southern Ontario where there are many ( the most?) greenhouses electricity is primarily produced by Nuclear (50..55%) Hydro power is about 24 ..38% of the total.

I think the bigger difference is the Canadian attitude about the "commons" nature of electricity and so profiting excessively on power is frowned upon.