Although I agree that they should have put money toward something else (I think moving on from badly received movies is better than revisiting them, even if I personally like them), I can see why they decided to do this:
1) The fan base was already there (how big it is is debatable), so that means they can build on interest better than they could do for any completely new show.
2) Covid meant that JL was in a good position because most work could be done without Covid restricting the progress. Also, the fact that most of it was already shot meant they had a relatively quick turnaround, they didn't need to write/cast/shoot a whole new show or movie with the unpredictable lockdowns coming from the pandemic.
And, i could be wrong as I don't live in the US, but from what I know there isn't a free trial for HBO MAX anymore since they announced all movies coming to the streaming service?