When DC's pre-Vertigo suggested for mature readers titles come up, the common narrative is that the "mature readers" label was used for characters who did not fit within the confines of the superhero genre. John Constantine. Swamp Thing. Shade the Changing Man. Morrison's Doom Patrol. All of these properties were subsequently folded into the Vertigo label. However there were more super-heroic franchises which also fancied themselves "mature readers" which did not graduate to Vertigo for whatever reason. These consisted of, The Question, Deadman, and you guess it: Green Arrow.
Now the reasons for the Green Arrow monthly not transitioning into the Vertigo imprint is evident. Some might say that Green Arrow was too rooted in superhero trappings to be a Vertigo title, but this is a faulty hypothesis, as the Animal Man ongoing had never been considered to be "mature readers" and was more unabashedly a part of the superhero genre than Grell's Green Arrow ever was, happened to be one of the select few titles that turned Vertigo.
I suspect a more plausible reason for Green Arrow not getting the Vertigo treatment was because Mike Grell was in the midst of ending his run on the character, and DC editorial had more plans of making Oliver Queen drift away from Grell's gritty and restrained interpretation into something more mainstream.
However, it really is interesting to think about alternate scenarios in Green Arrow became a Vertigo exclusive character. There are so many possibilities for how it could have turned out and the questions that come with it:
Who would be the creative team?
Would other Vertigo characters cross over?
Would it not have been cancelled due to lower standards for sales for vertigo?
What changes would be made?
Would Queen still have died and replaced by Connor?