I think series has pretty much captured the young adult market, I can't remember the last time someone recommended an actual standalone movie that wasn't a die-hard Marvel fan.
Edit: Everything Everywhere I think a was the last one.
Not least, young adults are also probably the least able to actually shell out nearly £20 to go and see a film. Younger kids get taken by parents, older adults might get a living wage to afford it. Plus, in car-centric places, it's hard to even get to the cinema if you don't drive a car. Meanwhile Netflix is probably are already subscribed to, so it's "free", and it's right there at home.
Edit: Everything Everywhere I think a was the last one.