Besides, Rousey lost the weight to compete at 135 and now is no longer a 145 fighter. It would be like Penn jumping up to WW at this point. Cyborg turned down the fight at 135. Nothing stopped her from conditionally accepting upon weight cutting attempts, or even attempting to shed muscle (there are ways). It's up to Cyborg, but one thing is clear is that 145 is not where Rousey will go. If Cris wants the fight, she has to go to Ronda's realm.
Also, The UFC could make a 145 division, but having a 115 division makes more sense if they are going to roll out a division at a time.