Its not just that. Road races are generally at circuits, which have all in one facilities. TT’s often at circuits or village halls, again with facilities.
MTB involves the middle of a field / forest, custting back trails often need landowner and FC permission so a lot of effort, catering facilities, portaloos, timing and marshalls. Hadleigh of course I expect does not come cheap, same for Ashton Court (Octoberfest cancelled after this year).
Its a high effort investment sport. I’ve researched some of this as I’m thinking of creating an event next year, in the SE/London yet on a ready/half made facility to reduce costs (watch this space) and the backing of club-based Marshalls and some local help.
Its is not easy, and even that will probably mean several days off work near the event for course prep/preriding.