DNF Duel is rather light on content in comparison unless you like grinding online matches for cosmetics.
It's just that the game isn't that good.
TL; DR: Brand name and audience are matter.
Someone doesn't know about early SF5 release.

Original SF5 literally only had 16 characters at release, same amount with current DNF Duel. Lackluster single player, no arcade mode, story mode for each character is 3 dumb AI matches.
Game also locks some outfits recolour behind in game money, called Fight Money. Fight Money is earned through completing story mode, or gimmick survival mode and multiplayer games.
Game didn't even rollback netcode, horrible loading times. But the rollback netcode is added later and horribly implemented.
Unlike SF5 and Granblue Fantasy versus, GG Strive and DNF Duel actually have proper rollback netcode.

And then there were bunch of problems people complained about fighting mechanics in early years, I don't play fighting games, so I'm not commenting on that.
Total game sales over the year,
2016: 1.4 millions.
2017: 1.7 millions. 2018, no data, although this was the release of Arcade Edition.
2019: 3.7 millions
2020: 4.5 millions, being Capcom 10th best selling title. Release of Champion Edition, but still missing the latest (season 5) characters.
2021: 6 millions.
2022: 6.2 millions.
Pretty much, the early couple years weren't that great. Roughly from 2018, game becomes more decent.
Trivia: DNF, actually has an Japanese anime adaptation 2009, Arad Senki, (loosely adaptation) and
two more Chinese faithful adaptions. Translation: the game is more popular in China than Japan.