The Rabbonon in the Mishna explain that a shliach cannot be zocheh in a gett for a woman because it is a chov for her to get divorced, but can be zocheh in a shtar shichrur for a slave since it is a zechus for him to go free. Tosafos quotes from Yerushalmi that a slave of an important officer would rather be a slave than go free, so it would be a chov. Similarly, a wife of one who has a bad "mum" would rather be divorced than married so it would be a zechus. The Yerushalmi offers a cryptic answer and Tosafos doesn't explain it very clearly. However, the Tosafos Ha'rosh explains that the Yerushalmi is really only coming to answer the question about the slave, why do we always regard it as a zechus? Because the officer can always sell the slave to someone who is not important without consulting the eved, therefore the eved prefers to be free rather than be in a state where he stands to possibly be sold to a unimportant person (since then it would clearly be a zechus to be free). The question about the woman is not answered by the yerushalmi, but Tosafos Harosh explains that we rely on the gemara Yevamos 118b that we always assume a woman would prefer to be married than single.
There is a difficult Ran at the beginning of the 6th perek of Gittin that indicates that we never really answer the question about whether it is a zechus for a woman to receive a gett when there is a brother, to avoid yibum. The Beis Yosef (E.H. 140) explains that although the gemara seems to conclude that it is still a chov to get divorced, the Ran explained the gemara was just rationalizing the tzad that it could be a zechus, but ultimately remains an unanswered question.
The Ran in Yevamos at the end of the 15th perek asks, why don't we just ask the woman if she wanted a divorce, and if she answers "yes" then it is a zechus? He answers that something which is assumed to be a chov, is considered a chov even if we later consult her and she claims she would want it. The implication is that divorce is always deemed an absolute chov, therefore her claim that she wanted it, is not believed. But, in Pischei Teshuva (E.H. 140:5) he is mesupak in a case where a woman appointed a shlicah l'kabala to receive the get, but the husband handed appointed a shliach l'holacha, would he be able to be zocheh for her since from her appointment of the shlicah l'kabala it is clear that she wants a gett.