Follow the Leader: It's no secret that the service came to life directly off the success they had with their shows on Netflix and wanted to follow the example of "cutting out the middleman" when it comes to original content.
The Arrowverse from The CW shows are especially popular on Netflix but the catch is that WB doesn't outright own the CW, they split ownership with CBS down the middle. By making shows on something WB owns they're cutting out two middlemen, Netflix and CBS. Though the Arrowverse is very lucrative in its own right and isn't going anywhere anytime soon
Killer App: The big draw for the service, apart from having a massive backlog of DC material from animation to live-action, to comic books, was that it had shows exclusive to the service. Young Justice: Outsiders, Titans (2018), and Harley Quinn were the first among them. The former two are especially notable for being former victims of Screwed by the Network, so this was likely what they had in mind.
Screwed by the Network: The transition from a video and comics streaming platform to a comics-only platform came from above, WarnerMedia that is, since they planned to bolster the all-encompassing HBO Max instead of maintaining niche services. Production of DC Universe's shows that are now housed and continued on HBO Max should remain unaffected, however.
but Fox had the TV rights for Batman due to Gotham and wouldn't allow it.