With the exceptions of Legends of Tomorrow in 2016 and Riverdale in 2017, The CW hasn’t had any big new ratings hits even by the lower CW ratings standards.
The CW hasn’t had the most success in the last couple of years when it comes to new series.
With a Stranger Things name attached, perhaps even viewers who usually don’t visit The CW would tune in.
Additionally, the network lost long-running series The Vampire Diaries, Reign, and Beauty and the Beast in recent years.