BTW, its worth noting that there are distinctions to be made between kedusha, kadish, and other parts of davening that require a minyan of MEN al pi halacha, and other parts of davening that do not, and the fact that men only lead those other parts is likely based in minhag and a vague sense of tznius (not clear cut halacha of tznius, but the feeling of what is and is not appropriate). This is why, in some ORTHODOX shuls, rabbonim have allowed so called “womens minyonim” to take place, where women daven together, with one of them leading the tefilos, but without saying anything that can’t be said without a halachic minyan. In those kehilos, the very real feminist pressures are such that such accommodation were necessary to avoid greater harms. – But the need for accommodation did not justify any changes in actual halacha.
So perhaps OP should reform the question:
Accepting that a minyan is defined halachicly as a body of 10 adult males, why can only certain parts of davening only be said with a minyan?