>>283093962
People have had this fetish for at least a century. (3DPD is relevant to prove this point)
Some brave souls snuck muscle girls into series hoping to get away with it. When these characters, rather than being viewed as "weird" or "unacceptable," got massive followings instead, a cultural shift occurred. The waters had been tested, and the demand for muscular babes was found out to be high. Now they're introduced into series in order to DRAW popularity to it, rather than being tentatively added due to fear of unpopularity.