What motivation could they have for these changes? It just seems weird because it brings no advantages to the table and breaks things.
YouTube did the same thing a while back; they initially used the native player but then switched to their own which is nowhere near as good.
The same motivation that any modern corporation has, saving money, especially one saddled by debt, and with roots as a public utility that didn't have to worry about user experience.
Some mid-level VP probably saw a chance to save some money by making the team that developed the old HBO app redundant, and reassigning the task to another existing team. like the one that's been producing the horrible AT&T TV apps.
Money can be also saved by moving the app to cross-platform development tools that ignore native features and APIs, but allow for a single code base to be shared across all the different devices where HBO is offered.
This is often most evident in the crappy desktop versions of mobile social media apps, like WhatsApp, where the companies just use something like Electron, which makes them fat and slow, instead of making the effort to write native apps for each platform.
Everyone was waiting to see how AT&T would ruin HBO, and that's playing out as expected. Classic Big Blue.
