She just doesn't want to.
It's not because she's hiding out from a potential superfight against Cristiane Justino, as fellow female fighter Marloes Coenen suggests, but rather because the "Rowdy" one has no faith in the drug testing administered in the sport of mixed martial arts (MMA).
From her conversation with Sherdog Radio:
"I could, but I'm choosing not to because honestly, I don't trust any of the drug testing. I've been doing this a very, very long time and I was in the Olympics with the most stringent drug testing in the world. You know a lot of those people are doping. There's new advances in drug testing every single year because people find new ways to cheat. The only way I can really feel like it would be a fair fight is if it's down at 135. If she's doped to the gills and making 145, it's not really that far-fetched for me to think that someone not on steroids could move down a division."
After compiling a perfect 6-0 record, one that includes six straight first-round submission (armbar) finishes, Rousey has all but cleaned out her division, barring a sudden influx from Invicta FC fighters. Her effortless disposal of Sarah Kaufman back in August left fans clamoring for a showdown against "Cyborg" in 2013.
Right after Justino satisfies her suspension for steroids.
Until then, the only fighting they'll be doing is through Twitter. But do these latest comments do anything to sway your opinion on which athlete should change weight classes to make this fantasy fight happen?