All about the money!
MM I beg to differ, the VOR is quite expensive to install and maintain and a lot of them are coming to the end of there lives.
So as the system is just a back up the most cost effective replacement for a VOR is an NDB it will do more or less the same job for en-roure navigation and if you put it on an airfied with a DME you do two jobs, an approach aid for GA and the DME and also helps the airline guys with FMS.