Because it's the most UNIX-like Linux distribution.
No dependency handling by pkgtool, which is wonderful.
Compiling all third-party software is a great way
to optimize the functionality of the applications and to have
a non-bloated system. A full installation gives you all the development
tools you could ever dream of. The fact that it's shipped with vanilla
kernel is another thing that I really love.
If you are a CLI power user, or a sysadmin, this is the distribution to run.
Not a coincidence it's the oldest surviving Linux distribution.
Patrik is doing a superb job!