Things selling out really quickly does appear to be a problem. It’s not just mountain biking, online entry for popular fell races is also pretty quick.
Not having entered is it simply first come, first served? If so then they could go along the lines of the Three Peaks where a certain proportion of entries are reserved for women, first timers, etc. and leave the entry process open for a week, the names go in a ballot then chosen at random. Cash is only taken at that point. It’s more work but possibly fairer and might stop the “I’ll enter just in case I want to do it” types who then never turn up.