Going the Venice/Toronto + early October route seemed like a good bet early in the year when this appeared to be a comp to Gravity and The Martian but seems awfully questionable now. The positive reviews it did receive post-festivals were almost completely overshadowed by the inane flag controversy and it's now sandwiched between what will likely be three of the top five October releases of all time. It's the very definition of lost in the shuffle. In retrospect a big tent, more explicitly patriotic marketing pitch at Thanksgiving, Christmas, or next July before the 50th would have been a smarter play.
That said, popular movies find an audience regardless of when they're released and clearly people didn't like a lot of this film. Gosling's inscrutability, the sometimes maddening shaky cam, the cold tone, the predictability of the kitchen scenes -- I think First Man works despite these criticisms and will give it a second theatre viewing. But clearly others were bothered.
The last, and perhaps main, reason I would give for the failure has been observed in Variety and other spots: the moon landing seems small scale at this point in cinema history and the marketing never overcame this. The millenial audience does not grasp the import and the baby boom audience is busy watching ASIB. The true follow-on to Apollo 13 was Gravity and First Man, fairly or not, seems like low stakes.