Alot are community projects produced by people in their spare time, and a few (openSUSE/SLES/SLED, Fedora/RHEL) are backed by large commercial entities who make money selling services and support. Canonical is the only one giving away their commercial distro for free... (openSUSE and Fedora are slightly different from their SuSE and Red Hat cousins)

Personally I'm partial to openSUSE, but I certainly don't fault Canonical for the way they choose to do business.