I don't think that is even close to being a correct statement. Jose Aldo is getting more and more popular all around the World. Casuals are really starting to notice Aldo and he is on the verge of reaching the class of GSP and Anderson. Not just that, then numbers were good for both Fox events featuring Nate Diaz who also has a pretty good following. I think you are more in the minority then you think. I think you would have a better argument if you adjusted your statement to include LW. I can see that more since many people still don't know more then 2 fighters below LW. But you have plenty of well known fighters at 155.
I do agree to some degree that LW is a step above the ones below it. I think it goes WW + above, then LW, then everything below LW. But even LW has had trouble drawing, hence why Bendo/Diaz was on TV. I know Cain/JDS was on TV, but that was the first fight so it had to be something huge (originally supposed to be Lesnar vs. Overeem).
Aldo/Bendo drew like crap the second time around, and the first time it drew not so good even with a strong overall card. Edgar/Maynard WITH Aldo title fight and a Sonnen fight couldn't get the same as Rampage/Hammill. I think that shows the drawing power (or lack there of) of the lighter classes.