Omicron has a heavily mutated spike protien. This makes antibodies generated by exposure to other variants, or by vaccines created for them, less effective at grabbing and holding the virus. Boosters compensate for this by greatly increasing antibody levels…although the antibodies are still less effective, they can swarm the virus in greater numbers.

Omicron does not evade t cell immunity, so two shots will still reduce your risk of getting really sick, but the third helps protect from mild illness and onward transmission too. Not perfectly, but substantially.

Origins have nothing to do with it unless you think there is a lab somewhere cranking out COVID variants.