HBO used to have live events even for the west coast viewers. The only tape-delayed events were "Boxing After Dark" cards. But now, even "World Championship Boxing" cards are tape-delayed on the West coast.
They must want it to come on at a certain time, if you're talking about the fights from overseas we never get to watch anything live.
HBO offers HBO East which shows everything live. That way fans get to see it live, casuals get it in prime time.