Mass financialization of studios and a need for high returns on investment to shareholders. That's it. You're not allowed to take risks as a showrunner or writer, because that might not pan out for the investors, which means you can't really make art because you're trapped in a formula that's been done so many times that there's legitimately no way to do anything new or interesting. And you can't do anything original because there's no existing market for it, so you've got to adapt something that already exists.

They won't finance more than 8 episodes (looking at you, Amazon), so you have to condense a storyline that won't fit into 8 episodes somehow, and you do that by hacking away at the source material until it's barely recognizable. In the process, every writer working for you is desperately trying to leave their mark on the final product so they can get some amount of creativity out of their systems, so now you've got 80 new subplots that they're practically demanding get in the show when you already don't have time to tell the story that already exists.

The end result is a mishmash of nonsensical plot elements, characters with no arcs (because there's no time to have them resolve their flaws, and focus groups show that viewers prefer characters once they've resolved their flaws so we'll just remove that element entirely!), abysmal pacing, and a final product that costs $300 million and looks like utter shit. Rinse and repeat until even the most treat-brained of viewers decides that this is a road too far and stop paying for the streaming services, at which point the whole industry will collapse under its own rotten weight and new TV content will be exclusively reality TV or gameshows.