Would appear that Stephen Amell told the producers towards the end of season 6 that he felt for personal and professional reasons that the program should end after season 7. His original contract was for 7 seasons I believe. They managed to persuade him to do these extra 10 episodes to finish the program properly, but did consider continuing the program after Stephen had left, but decided against it and thus it was canceled. It is possible that an Arrow spinoff may occur though, possibly with Mia as the lead.