Because Ronda's star is on the rise, and Cyborg is so scared of being armbarred/drug tested she wants to fight at a higher weight.
Rousey can keep subbing hopefuls without enough training in the meantime, she will do well with or without a Cyborg fight on her resume.
Cyborg on the other hand has been disgraced for failing that drug test. Some say she needs to be gender tested too
The onus is on her to cut weight if she wants to prove herself and get her honor back. She needs a big win without drugs if she wants to get back in everyone's good books.