Well, they're not really creating a distro from scratch in the way that Debian maintainers pull source code from various upstream projects and spend weeks to months doing bug testing. The RHEL sources that the SL folks pull have already been bug tested and verified to work very well. The SL folks are compiling those sources (with their own modifications). Of course, you could do this with the Debian or Ubuntu source code, but I guess they figure if you're going to get source code from somewhere, it might as well be a well-known enterprise distro.