Why is Arrow coming to a close after season 8 and over 150 episodes while the rest of the CW's superhero shows are continuing on? There are a variety of reasons but most of them basically boil down to it being time for the series that started the Arrowverse and the story of Oliver Queen to end, so that other series can begin and the Arrowverse itself can enter a new age.
When Arrow first aired in the fall of 2012, its showrunners had a loose five-year plan for the show, presuming it made it past its first season. That plan began to change drastically when Arrow proved to be a surprise hit for CW and plans for the first spinoff series, The Flash, started along with Arrow season 2. By the time season 5 started, Arrow was the centerpiece of the most ambitious shared universe in television history and there was no way to move forward with the original plan for the series' conclusion - ending it as it opened, with the shipwrecked Oliver Queen's escape from the island of Lian Yu.
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Circumstances have changed over the past two years, however, and Arrow is no longer the critical and commercial darling it once was. The Flash is now the highest rated series on CW and the rest of the shows making up the Arrowverse - Legends of Tomorrow and Supergirl - are not at all dependent on Arrow as a starting point for new viewers. Ironically, after helping to prove that superhero shows could be successful and done well with a television network's budget, the more down-to-earth Arrow is no longer needed to indoctrinate newcomers to more high-concept shows like Constantine or Black Lightning.
Beyond the larger multiverse that Arrow created, the story of the show and its main hero are all but played out after seven seasons. Stephen Amell, who plays Oliver Queen, said in an interview on Michael Rosenbaum's podcast, Inside of You, that the only thing that truly remains for his character to accomplish at this point is "he needs to leave a legacy." While it could be debated that Oliver has already done this in several respects (such as by siring two children, who just met in the future of Star City 2038) it remains to be seen if the legacy of superheroes he inspired will be able to build a better world in any of the various foretold futures of the Arrowverse.
With the modern-day Arrowverse now counting down to Crisis on Infinite Earths as its annual crossover event this fall, even the immediate future is looking grim for Green Arrow's legacy. Yet if it should be Oliver Queen's destiny to die a hero's death saving the multiverse (as many theorized even before it was announced that Arrow was coming to a close) it would be a worthy end for the hero who started it all. It would also offer the CW the clean start they desire for a new phase of DC TV shows, perhaps with the upcoming Batwoman series as a centerpiece.