Hmm. I wonder why?
It's long been known that while the CW/DC universe does have access to a wide array of DC Comics' roster, if plans are in the making for a movie with those characters, they have to duck out of TV appearances pretty sharpish. This isn't new for Arrow, especially. Deathstroke's absence after his initial arc of the show in season 2 was down to the fact that Warner Bros. was developing plans for Deathstroke's involvement in the DC movieverse. Then the show's take on Task Force X had to abruptly vanish when the Suicide Squad movie ramped up, and that's after Warner Bros. let Arrow use the characters only as a test run ahead of the movie.
It had seemed, at least in Deathstroke's case, that those restrictions on the show had waned: after all, he started making more appearances and then returned in full earlier this year before returning again for another major arc in the first half of season six. But now that that storyline is over, according to executive producer Marc Guggenheim (speaking at the recent ACE comic con), the show is back to having Deathstroke off-limits, thanks to the character's appearance at the end of Justice League and the plans for his own spinoff film:
These are DC's characters, we're just renting them ya know, so we use the characters that they tell us we can use when we can use them, and we work around whatever limitations that places us...
... For a time they were saying 'you can't use Deathstroke', and that changed and we were able to have Slade Wilson back on the show, and now we're back to 'you can't use him'. They're working on the feature film version, it really is sort of now that you've got obviously Slade, spoiler alert, Deathstroke appears at the end of the Justice League movie.
Given the precedent for this situation, it's not a surprise, but it's still a shame that Arrow has to get moved around with what it can and can't use because of the movies. At this point, considering the Arrowverse has been around over five years now, I think people can accept that the TV and movie incarnations of these characters are in different universes, and they can exist side by side with each other — look at The Flash, for example! But alas, it's not to be, at least for Deathstroke. So say goodbye to Manu Bennett for at least a little while yet.