The most important patents, like this one : https://patents.google.com/patent/US4455675A/en

Have expired for a long while. It just took some time for serious competition, like the Sony MDR-1000X and its descendants, to come out.

Those patents are really old actually. Noise cancelling was unknown to the general public prior to the first quietcomfort headphones from Bose but it had been in use for decades before in Bose aviation headsets.

Well they are still the leaders but the patents that are now expired give good-enough of an experience for most regular users.