While not officially stated, there's an obvious reason why Netflix and Marvel are parting ways: Disney+. The service's late 2019 debut is fast approaching, and Disney hasn't been shy about wanting to use Marvel content as a draw for subscribers. Like it or not, Netflix's cancellation of its Marvel productions helps Disney avoid the awkward position of supporting exclusives on a directly competing streaming service (Disney has a stake in Hulu, so it has an incentive to keep Runaways going).
Netflix already has ideas for what to do next. It has plenty of superhero-themed content on tap, including the just-launched Umbrella Academy, and plans a string of movies and TV shows based on Millarworld's comics. Still, there's no denying that this is the end of an era for Netflix. It started working on Marvel shows at a time when originals were a novelty on the service, and major franchises were practically necessary to keep people interested. Now, it's enjoying massive success with originals on a regular basis. As good as the Marvel programs have been at times, they just aren't as essential as they once were.