All new incoming Western fighting game players being indoctrinated into Capcom patronage at the start of Seventh Generation.
Lack of comparable graphical superiority to other titles of the time.
Older players dropping out due to life reasons or losing interest in new direction of franchises.
Being made almost exclusively in Japan during a time when HD transition was struggled with.
More Eastern flavors incompatible with Western tastes being instilled in Japanese multimedia.
*Some apply to 2D fighters as well.
**Soul Calibur II was well known and liked. Soul Calibur III was resented due to beta quality (on PS2) and exclusivity. Soul Calibur IV had a mixed reception with content reduction and stiff and unbalanced gameplay. Soul Calibur V turned everything the franchise was known for on its head to cater to Capcom players.
***Virtua Fighter never took off in the West due to poor Western Sega Saturn debut and a cheaper alternative in the arcades and consoles that gained notoriety for its presentation (i.e. Tekken). Virtua Fighter 4 was a cult hit in the West, but 5 fell flat due to lack of online in its PS3 debut and lack of casual content. SEGA’s shrinking arcade and console software market kept them from updating hardware which the franchise was historically used to show off. The departure of the head people from the franchise’s past was also likely a factor.
****Dead or Alive in general is niche due to its risque direction. Dead or Alive 2 was impressive for its time and available on a variety of platforms so it was well critically recieved and garnered sales based on that alone. The future sequels prior to 5 were only available for Xbox owners, largely without interest in fighting games so their market was limited. Dead or Alive 5 was poorly received on its debut from fans, but according to some sources Dead or Alive 5: Ultimate hit a new sales milestone for the franchise. There has not been much enticing new players after that.
*****Tekken Tag 2 sold far below expectations and it falls upon Tekken 7’s Western release to determine the investment Bandai Namco will put into the future of the franchise.
******In all cases new iterations are necessary to keep the franchises going, but large investments from parent companies into franchises outside of Tekken have not been forthcoming.