I believe for DC it boils down to sales & pre-orders
For example, for Superman they released one volume of Superman: Blue in 2018 (which was listed to be the first of 4 volumes
https://www.penguinrandomhouse.com/b...by-karl-kesel/) but no other volumes ever materialised
However they have successfully released near complete run of Wally West's Flash series from issue #62 to #225 across 14 books (the Flash by Waid/Morrison & Millar/Johns) as well as a volume at the start of the run (The Flash: Savage Velocity of issues #1-#18) so hopefully that sold well enough to get the rest of the run (#19-#61 plus the post-Johns issues of #226-#247)
Likewise a large amount of Wonder Woman volume 2 and Batman post-Crisis has almost been completed