The problem is with the interpretation of the term papa-yonih in Gita 9.32. Should we interpret it literally as a sinful womb? The answer is no since such an interpretation will make Lord Hiranyagarbha, the ultimate source of both man and woman, sinful.
He, verily, had no delight. Therefore he who is alone has no delight.
He desired a second. He became as large as woman and man in close
embrace. He caused that self to fall into two parts. From that arose
husband and wife. ....
Brhadaranyaka Upanishad I.4.3
Should we interpret it as socially inferior? We have to be careful about this translation also because of Gita 9.29.
I am the same towards all being. None is hateful, and, none dear to
Me. But those who
worship Me with devotion dwell in Me, and I too dwell in them.
God does not view people through the social lens since all people are same to Him. So the correct translation of papa-yonih is socially oppressed. Vedic people did not allow women, Sudras to access the Vedas. Krishna is thus referring to that oppression in Gita 9.32.
A complete discussion of Gita 9.32 would require also an explanation of Gita 9.33.
Then how much more so in the case of holy Brahmanas and also of
devoted royal sages! Having come into this impermanent and unhappy
world, engage yourself in My worship.
Why will the Brahmanas and royal sages find it easy to gain the highest spiritual goal?
The simplest answer is that they did not experience the social oppression faced by women, sudras etc. This shows that translating papa-yonih as socially oppressed makes perfect sense.
Why didn't Krishna just say that all will attain moksha?
He did in Gita 9.29 where he says that He is the same to all. He does not distinguish between high and low born people. Such social distinctions mean nothing to Him. Gita 9.32 and 9.33 are simply applications of the position stated out in 9.29.