I'm curious - for what do you use SL? I had tried it at v4 and it did not seem superior to a 'standard' Fedora install at that time, even for scientific analyses (ROOT and GEANT were no different to install IIRC and there wasn't the same healthy package system that comes with Fedora). Has it improved in some way since then?
It's an historic left-over. My company uses CentOS, and back in the days of the transition to v6, CentOS was delayed. SL6 came out at the right time and we switched.
SL is basically the same as CentOS: a community version of RHEL.