Because they were stupid in ever removing the "HBO" brand. The HBO brand is associated with quality. Sure maybe it's too "prestige" and scared off a few people, but you've weakened the brand by just calling it Max.
Plus, the rebranding failed anyway. No one calls it Max, it's just HBO or HBO Max (which only stuck because of the confusing and inconsistent HBO Go and HBO Now branding).
The sooner David Zaslav is fired the better.