Call of Duty: Vanguard was met with rather mixed reviews at its debut and the community wasn't particularly convinced either with lots of criticism of how trivial and inaccurate World War II was portrayed, while others were upset that it was far too "woke" and still others over the illogically jumpy story.
Initially it didn't seem to sell very well and Activision themselves wrote in a report that "the game's World War II setting didn't resonate with some of our community and we didn't deliver as much innovation in the premium game as we would have liked". But since then, things seem to have been going well, as a former social media strategist for Activision Blizzard reveals on his LinkedIn that the game sold "30 million copies".
A reasonable guess is that the World War II theme that Activision Blizzard initially blamed for the lack of success made Call of Duty: Vanguard stand out from the crowd and that it continued to chug along for a long time - even if it was slow in the beginning.