AT&T’s mobile service currently doesn’t incur bandwidth for streaming HBO services (Max, Go, etc). When customers are choosing a streaming service, chances are they’ll gravitate to HBO instead of a competitor if they expect to be streaming heavily while on-the-go and on a shared data plan (or even an unlimited data plan - HBO streams won’t contribute to the ~22GB/mo of ‘unlimited speed’ data you get on the unlimited plan that isn’t throttled).