Money? The spirit of audax and randonneuring prevents sponsorship of events and participants, with a couple of exceptions (PBP has event sponsorship). The running marathons of which you speak generally have sponsorship, and coupled with volume of participants paying entry fees, the budgets for promotion are considerable. I gather that the double century events you mention are run somewhat outside the audax/randonneuring philosophies (nothing wrong with that). To me UMCA is the pivotal organisation in running the types of events you discuss, which seem to me to be more races than the esoteric randonnee-style events.
There are hallmark randonnee events all around the world that attract hundreds of participants for each edition. In Australia, there is the Alpine Classic that is generally oversubscribed months ahead. In South Africa there is an event that draws 14,000 people. PBP has its 4,500 (maybe 5,000 next year). Some people, like me, also prefer the more relaxed and friendly nature of smaller events.
There is a place for everyone and every event.