Stranger Things doubly suffers because it's horror. In the first season, neither the characters or the audience know what's going on. The monsters are new and scary. The concepts are new and scary. The first season is incredible because it's all unknown, and because there's an almost cosmic horror quality to it.

However, by the end of the first season, both the characters and audience are experienced. The monster has been revealed and killed and, while it was tense and scary, the characters and audience know what to expect next time. The upside-down has been revealed and, while there's a lot about the idea left to explore, there's and understanding of what it is, how it works to some degree, how it's linked to the real world, etc. Everyone has knowledge and experience. And with knowledge and experience, the horror dissipates.

So where do they go from there? Well all they can do is to make bigger, scarier concepts or to throw more of the same at the characters. More of the same can make for good action - see Aliens - but the horror element just doesn't work any more, and it loses a sense of intimacy that a single monster brings. So the only way to try to maintain that feeling of horror is to go bigger and scarier.

Of course, the issue of intimacy remains. How do you have a huge, scary monster - far bigger and scarier than the first one - while still keeping it feeling both personal and intimate to our characters and having it feel "beatable"? And, well, you can see how Stranger Things struggled with that in season 2.