Now that I've thought about it a little more, I came up with another reason: I rarely actively find questions to answer on SE (I usually answer questions I come across while trying to solve a problem), and while programming research almost always ends up with me on SO, sysadmin problems find me on NixCraft, linuxquestions.org, and (mostly) a wide variety of mailing list archives. Developers and sysadmins seem to look for help in different places, in general, and thus the body of knowledge differs in location as well.

@Iain I consider myself a sysadmin and yet have three times as much rep on SO as SF. I think this comes from a few things: I do program frequently, I've been on SO longer (as it's older), and an awful lot of questions end up on SO from programmers trying to do system administration. There's also the subreddit effect - more upvotes happen on the more popular site.