Part of Yakuza's problem:
1. Dub (many people in my Yakuza gaf threads would say they aren't buying it because of the dub in the trailer and demo), and localization.
2. Poor publicity. I saw not one single commercial for Yakuza and I thought it could have done well with a modicum of advertising. When a game like Shadows of the Colossus sells the way it does, no way is a game where you curb stomp fuckers going to bomb with a decent ad campaign.
3. The name. The original name translated in English is far cooler, resonates with martial arts movie fans, and is more relevant than..."Yakuza", especially since, afaik, like in Yak 4, it doesn't star an entirely Yakuza cast.
Also, I don't think porting ten year old games is going to kill Sega but that's me.