People like options, especially the Linux/FOSS people. You could come to a consensus about something but it'll only go so far until some developer decides to fork it and make their own project :)
Yet another DE means extra set of conformance tests, bugs to fix, look-and-feels, supporting pages. These all mean money but your customers may not even care about this feature.
So let's say GNU/Linux can use whatever DE and it will always be. But RHEL is just a business product derived from it, with a selected "blessed" packages. There're no conflicts.