This is one of the most useful answers that I’ve ever received during Office Hours. My guess is that there’s something in Johns’s contract for the book that precludes it from being put on DCUI for a certain amount of time. There are other, older Black Label titles that came out in 2019 & 2020 (like Dark Knight Returns: The Golden Child, Joker: Killer Smile, Joker/Harley: Criminal Sanity [except for issue #1], and Wonder Woman: Dead Earth) that have been held off of here, probably for similar contract-related reasons, moreso than sales. However, the ones that started coming out in late 2020 and 2021 have all turned up here on schedule (I think), so I would take that to mean that DC reworked their standard contract terms with creators to expedite the arrival of Black Label books on DCUI.
To be exceedingly clear, all of the above is purely baseless speculation on my part, and it would be a good idea on your part to assume that I have no idea what the hell I’m talking about.