Certainly more popular than this.

Shenmue I sold ~380k in Japan on the Dreamcast. 

Shenmue II sold ~150k, but the game was released in 2001 when the Dreamcast was already dead and SEGA went 3rd party.

So in a span of 2.5 years from the release of the first game to the release of the second it lost around 230k sales.... and you're surprised that the 3rd game isn't doing well now 18 years after Shenmue II was released?

Dude, releasing a game at that time for the Dreamcast was akin to come out with a Wii U exclusive during the 2017 holiday season when the console was pretty much dead and replaced already. At that point reaching 150k is actually a big achievement, because everybody has already moved on with the console.

Dude, there were 380k people who owned a Dreamcast who bought the first game. They're still the main audience for the sequel, dude, which it completely failed to reach.

Sakura Wars 3, which launched for the Dreamcast in 2001, sold over 300k copies. What a shocker. Sakura Wars 4 came out a year later in 2002 and still managed over 250k copies. My god how could that be possible as everyone had already moved on.

Sakura Wars 3 came out in March, 2 weeks before Sega announced the console would be discontinued. Players left the console in droves, mainly to PS2, after that announcement. When Shenmue II came out in late November, the console was already dead and had little to no shelf space anymore in stores. The fact that a port for the OG Xbox got announced in the meanwhile probably didn't help matters, either.

Sakura Wars mainly survived due to it's renommée, something Shenmue didn't get the time build up

Shenmue

All but 7 games will be gone in a month.