Support isn't just about SLA's, its about your vendor actually providing software/drivers/etc for your environment. I think you will find many commercial applications only support recent versions of RHEL, SLES or in some industries Oracle Linux.
These products don't ship with source code, and they tend to fail spectacularly when put on machines that don't happen to have the "right" kernel, library and system tools. Frankly, having shipped binary products on linux in the past, it can be a real PITA just maintaining an application over three or four versions of RHEL or SLES (which have a lot more in common with each other than they do with debian based distros).
