This is always a popular topic on Omega forums.
I've read a bunch of the threads. No one really knows, but at the end of the day the watches are all (more or less) readily available from gray market dealers and authorized dealers confirm they hold inventory in the back for vip customers and never put them in the case. It's a problem Rolex could solve if they were motivated to do so. Clearly they aren't for whatever reason.
The only comparable situation I can think of is Ferrari (although I've been told there are designer hand bags with similar supply 'issues'), where you can't walk into a dealer off the street and buy a new 488 even if you bring a sack of cash and will pay in full on the spot, they'll make you buy a used car or less desirable model first and then maybe you can get something in the future if grow your relationship.