halacha - Why can't a mixed gender meal of 3 form a zimun?

In a group of two women and one man (or one woman and two men), why can they not have a zimun together?

Is it because of different levels of obligation in bentching? If so, why is a woman less-obligated (differently-obligated?) to bentch?

What other reasons have been given for this rule, which as far as I know is universally held and stems from the Mishna (Brachot 7:2)?

_{Closely related: Woman and Zimun: What is the requirement?}