Ironman no one has dominanted the LW divison either. That's why there are so many contenders. Sherk and BJ both have fought in a combined 5 title fights since 06. And that is counting there fight for both of them. BJ has fought 2 guys Stevenson and Sherk. Sherk has fought 3 Florian, Franca and BJ.
I'm not saying the LW divison isn't good I'm just saying the same reason there are a ton of contenders at 205 is the same reason their are a ton at 155. No one playing the role of Anderson/GSP and beating on all the contenders.