Re:
I think part of the problem in organising large national scale group rides is the lack of commitment from those attending. You put a large effort into getting everything sorted, in the case of National series rides there is much to be done.
Route planning - including route recces (may not be local to organiser) and ride guide route familiarity - the person at the back needs to know which direction the front of the ride has gone.
Liaising with the land owners for access permission - you can't turn up at a trail centre with 40+ people in a group and expect them to help you.
Organising lunch stops and parking - you can't turn up at a trail centre with 40+ people in a group and expect them to help you.
First Aid provisions - qualified first aider with proper kit.
Risk Assessments - required for insurance purposes and land owners protection.
None of this happens by chance.
No wonder these rides no longer happen.