Why? Because now is best time as any. Toronto already has some incredible pieces up front, if they feel like they'll be in the playoffs Green would be a huge asset for them, and not just for this year, next year as well. Worst case scenario, if you don't make the playoffs next year you can trade him away as a rental.
Also unlike 2018, the 2017 draft seems like a weak one. Given their current position, the Leafs are not drafting in the top-10 unless they free fall, and if they free fall all these trade-deadline deals are moot points anyways because they'll likely sell rather than buy.