Massively overdue. Why did they not do this the moment Don’t Ask Don’t Tell was ended?

Because homophobia. That and the military is a boy’s club.

Only my opinion, but I think DADT was such a massive failure and created so many problems that issuing upgrades to discharges concurrently would have emphasized not all Democratic ideas are flawless. Plus, social change in the military has always been one of immediate policy and slow acceptance.