A more than good enough reason can be made just by looking at the events of Diablo 3 RoS.
At the start of RoS, Malthael had sent his reapers to attack and destroy a great number of cities within Sanctuary; many people died during these events and had their souls harvested. Now keep in mind that this was going on continuously throughout the entire Act 5 campaign.
There’s also the possibility that Skovos Isles (The Amazon’s home) was also attacked by reapers. If such a thing did happen then there was no way they came out of it unscathed and as such could’ve lost a considerable bulk of their fighting force (which is once again composed of women only).
To make matters worse, near the end of the campaign, Malthael was using the black soulstone in an attempt to eradicate the rest of humanity (the black soulstone would absorb the demonic essence from people, tearing their bodies apart in the process), and while we indeed destroyed Malthael, we couldn’t stop him from murdering a horrifically large number of people with his usage of the black soulstone.
So in short, at great catastrophe would be more than enough reason (in my opinion anyway) for the many clans or groups to change how they handle things. For example, the Askari (Amazon) fighting force consisted of only women; that would mean that if the reapers did attack Skovos Isles, then the ones who fought against them were mostly women. And as I said earlier, there was no way the Askari would be able to fight the reapers without sustaining casualties, and most of those casualties mostly women.
Then during all the chaos, Malthael’s usage of the black soulstone also killed people numbering on a global scale, with Askari being affected by this as well; as such they lost even more people. After those events, the Askari (along with various other clans and/or groups) had lost a great number of their people (most of whom we could assume to be women, considering they fought the reapers).
So for them to begin training their men alongside their women for battle wouldn’t be that far stretch considering they had suffered a catastrophe and hope to prevent another one from happening again. Their oracles could even foresee a looming disaster that’s waiting to happen and come to a decision that their current military won’t be enough, hence the training of their men.
With all that said, I have to add that while I enjoyed the amazon in Diablo 2; I’m not really expecting nor overly hoping to see the class return for Diablo 4. My points were that if the amazon does return, then it wouldn’t be a blasphemy to the lore if a male variant was included to the class considering that the lore itself never stated that their men couldn’t fight or utilize their people’s techniques; and the Askari deciding to train their men in light of Diablo 3 RoS would be a reasonable change in my eyes.