Why? It's not specific to the WiX Toolset at all. Other projects can (and some have) adopt the Open Source Maintenance Fee with no changes (or they can change if if they want).

WiX is just the first project to use the OSMF because I need a project to "debug" an issues in OSMF system. As we get all the issues resolved, we may see the OSMF be adopted widely... or not.