Little guys aren't big money makers. Lets just state the obvious here. If you're under 155 and male, nobody is going to take you seriously as they do at HW LHW MW WW or even LW. Renan Barao could kick my ass, and most larger fans, but lets face it. This is a case of supply and demand. If nobody wants to see them or sponsor them or whatever, they're shit out of luck.
Just like a lot of people don't like WMMA (I personally love it.) Just like the WNBA is a complete joke of an organization. Nobody cares about women's basketball.
Now before somebody brings up boxing, and how small some of the legends in boxing were/are, boxing is an ancient sport. It has accumulated a huge enough casual fan base that even little guys are well respected.
It will take time for that to happen in MMA. Right now, casual fans of MMA want to see big or average sized men square up.