Sometimes it is as others have noted: Because you are promoting an internal candidate. So ya, the requirements are tailored to that person. This isn't a pure evil "Oh we want to keep anyone else out," kind of thing but that we already have a guy who is trained and qualified on the stuff we use. So if we are to consider anyone else, they would need to be as well. There is no reason we'd want to hire someone that we didn't know, and that wasn't proficient with our systems when we already have someone who is. However, we'll let people apply, on the off chance there is a more qualified candidate.
The other is as you say, needing someone that can hit the ground running because we don't have a ton of skills overlap. We have few IT people and a lot of systems, so we can't all be good at everything. I'm sure there are some arrogant Slashdoters who've never worked in an enterprise that think they can be all things to all people, but you quickly discover that isn't the case. So when we hire someone, we need, or at least strongly desire, certain skills.
Like our last UNIX guy we hired. They had to be good with Linux, since we've been moving all the UNIX stuff to RHEL. However we still had some old Solaris SPARC shit around back then (gone now thankfully). It was running important things, and we couldn't just turn it off. So we really wanted someone who knew Solaris. Not "Oh I know UNIX and I can learn the differences of versions, given time," but someone who could dig right in when one of those POS's went down and needed to be fixed RIGHT NAO!!! So we wanted, and got, an older guy who had a wide range of UNIX experience, including Solaris, rather than someone who was all Linux, all the time.
While learning is great and is required at any IT position, when you have a small team and are looking for a senior position, you don't have the luxury of bringing someone on who doesn't know the technology you use but wants to learn, since they may well be the guy in charge, and needing to support it all right away.
When we hire a student (I work at a university engineering college), we are looking for brains and ability to learn. Minimal experience is no problem, they can learn and indeed we expect that's part of the reason they want the job. When we hire a UNIX lead, that guy had better have some experience on the stuff we use because he'd going to need to be able to do it from day one.