It's a statistical fluke that has been observed with Delta as well, particularly with the AstraZeneca vaccine:

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3949410

It stands to reason that the unvaccinated at this point are in large parts hermits who are "protected" by their lifestyle. However, it's also plausible that the non-neutralizing antibodies from the vaccine induce a suboptimal immune response with Omikron.