Conventional wisdom is that the Greens are sapping off support from the three centrist parties (some would say center left, but those are people with short ideological memories). By excluding the Greens, May is is able to project the Greens as a party of protest with fresh new ideas that the other tired old mainline parties are afraid of. Of course if one trolls through the green platform there really is not much novel there. A leaders debate would have been the perfect place for the other three parties to make that clear. For the Bloc and the NDP it would have been a particular bonus because unlike the liberals they would not have to talk about what they should have done and at times promised during 10 years in power. That is the Bloc would have had an opportunity to showcase an environmental platform that was good for Quebec and the NDP could have demonstrated its long time commitment to environmental issues going way back to the 1970s and even longer if one includes workers health and safety.
Sometimes turning a challenge into an opportunity is the genius of politics. Playing it safe is the menu de jour.