Having been on the hiring side, most of the time it's because it's a maybe. There's probably an overqualified candidate who could work somewhere better and you.

Or in some cases, sometimes there's nobody else but you. Startups have very low application rates and often with an active hiring period, there's still nobody who qualifies for the job. Four times I've seen a company go with a "last resort" applicant after 2 months of interviewing everyone else. Come to think of it, I was actually rejected for my current job but they actually put me on file when they said they would and hired me 3 months later.