soul calibur 5
UFC undisputed 3
Street fighter X Tekken
Skullgirls
Tekken Tag Tournament 2
Dead or Alive 5
Persona 4
Playstation Allstar battle royale

Those are the reasons why.  I've done no research and i'm just being placing this as an opinion, but I put on low numbers due to those listed above.  I have a friend that's all about fighting games.  He wants to go pro.  But, even he stopped at Tekken Tag Tournament 2. 

He'll be getting PABR as his last fighting game this year.

http://www.siliconera.com/2012/10/29/hold-on-to-your-cowboy-hats-dead-or-alive-5-sells-over-500000-units/

DOA5 shipped over 580,000 units

DoA4 was already over a million on 360, but then ioi took off whole regions. It's not an adjustment, it's just a bug.

Haven't checked DoA5, but the 360 version was again not counted properly in its opening week.

DoA5 on the 360 only has the Japanese numbers.

Screw DOA, look at the Tekken Tag 2 numbers what happened to fighing games overall. SFxT, Soul Calibur sold pretty weak as well.

blame Capcom's DLC foolishness for SFxT sales, Soul Calibur 5 didnt really go over well with the fans, it seems alot of them didnt like the new characters or the story mode. The little bit i played at best buy didnt really convince me that i shouldnt just stick to part 4

Dead or Alive 2 is my favourite, and it only got more awesome with the Ultimate Edition for Xbox in 2004.