No one is ducking anybody. Cyborg just makes terrible decisions for her career.
Let me help you out here.
Rondas last fight at 145 was on Nov 18th 2011 against Julia Budd.
Cyborg fought on Dec 17th, that same year, against Hiroko Yamanaka. That fight was a NC due to Cyborg testing positive, and she was suspended for a year. The 145lb division was done after that happened.
You also might want to read this.
http://www.mmaweekly.com/cris-cyborg-unleashes-after-ronda-rousey-victory-looking-to-leave-strikeforce
and this...
http://www.mmaweekly.com/if-victorious-in-ufc-debut-ronda-rousey-will-likely-face-cris-cyborg-next
Also, to add more, Cyborg recently had the fight right there in front of her but turned it down. She asked to be released from her UFC contract. Said she couldn't make the weight, she would die, or couldn't have kids. Now that she is released, she has gone on record as saying she wants to cut the weight to 135. Cyborg makes some very weird career decisions. Why did she ask to be released from the UFC, when she had the Rousey fight right there in front of her face???? We could have already seen this fight go down, but Cyborg is the one that is making the bad career choices and preventing it from happening. Not Ronda Rousey. Cyborg just needs to make the weight and go from there.