Navy SEAL drama Six's cancelation was due to the military drama marketplace becoming "incredibly crowded" and "derivative"

Deadline

History channel executive Eli Lehrer said of the cancelation of Six after two seasons: “I think what we found is, between the end of Season 1 and the launch of Season 2, the marketplace became incredibly crowded with similar, one may say derivative, shows. When we came back for Season 2, it made it challenging for our series to reestablish itself. We saw a decline in the ratings as a result.”