because when several C-USA schools left to join the AAC, C-USA "stole" schools from the Sun Belt. Now that isn't to say that C-USA teams are suddenly not good. It has more to do with the perception of the conferences within the Group of 5. At one time the tiers were probably like this:

Tier 1 - C-USA, MWC, and maybe the WAC (when it existed)
Tier 2 - MAC and Sun Belt

Now the lines have become more blurry, so it may look more like this:

Tier 1 - MWC and AAC
Tier 2a - C-USA
Tier 2b - MAC and Sun Belt