In my case, it's my distro since always. I'm not at all one of those h4xx0r types, I'm just a lawyer, translator and theologian (and professor) doing my job with Linux. I started using Linux in May 2000 with a boxed version of Red Hat 6.2, then went to Red Hat 7.1, 7.2, then switched to Mandrake until 9.2 came about, and the dependency hell really irked me. So I searched alternatives. About 2003-2004 (not sure really) I began to use it. It was easy for me to configure it. I always used my Linux on laptops, at that time if I wanted to be online I had to setup a Winmodem, and maybe other hardware. That meant that even on "friendlier" distros such as Red Hat or Mandrake I had to tinker with the command line and config files.
Thus, when the time came, upgrading to Slackware came naturally. And I appreciated that it always was fast and lean, consuming much less resources than other distros. Now that's not so crucial, but in the early 2000's it was quite important.
Slackware was there at the right time, offering me what I needed, and it was fast and lean. And I like its simple approach to system maintenance; I can get a good grasp of the whole system.
Also, at the time Gentoo was just beginning and (again) I was using dialup Internet, paying by the minute, and I really didn't appreciate the prospect of compiling almost everything. Other distros (such as Arch) were also beginning.