There is a difference between a show being cancelled and a show ending. Supernatural has been on the air for 15 years. Not only would the actors genuinely like to move on, the network has to keep giving them raises if they want to continue. While Arrow didn't run as long as Supernatural, the same logic applies. The actors want to move on. Besides, if we're being honest with ourselves, Arrow began to run out of steam around season 4 or 5. Better to end it rather than let it continue as the quality drops further and further.