It was easy decision. The amount that they dedicated towards boxing has a much lower return than the amount they dedicate towards making TV shows. Boxing ratings had been declining for a while and for a variety of reasons people were not watching boxing as much
On top of all of this they were in comp with streaming services, showtime were putting out quality content, apple TV. So HBO stepped up and brought out stuff like WestWorld, True Detective, GOT, etc with big name stars. Boxing just didn't really have the combat stars to compare with these big movie stars. I really do miss HBO boxing so much.