Relationships in general make people a bit nervous. It's about trust. Do I trust you enough to go there?
I think its fantastic if your idea of fun is cuddling up on the couch with a movie or just relaxing together. However, I am not in the market for a total couch potato. If there is zero activity in your life, we will probably not be the best match.
People could rationally decide that prolonged relationships take up too much time and effort and that they'd much rather do other kinds of things. But most people are afraid of rejection.