There is no big reason why the movie Assassins failed at the box office..

The era when movies set in the 1970s and 1980s with political themes were a guaranteed box office hit has passed.
Over the last decade, we had 10-million-viewer hits like 12.12: The Day, A Taxi Driver (and including The Attorney, that makes three),
1987, The Man Standing Next, Hunt, and others that frequently hit the 5-million mark,
but since 2024, combined with the decline of the film industry, there have been no major hits.
The biggest reason is likely the exhaustion of subject matter,
and audience fatigue probably plays a role as well.
Seeing how it gathered 1.5 million viewers over the 4 days of Chuseok,
only to drop to 60,000, 50,000, and 7,000 viewers from the 5th day onwards,
it's also possible the movie was so poorly made that it couldn't even generate good word-of-mouth.