Actually, holy was very good throughout season 1 in TBC and for a while in season 2. Disc was only really viable in 2v2 in season 1. You probably recall the often quoted 27/33/0 cookie cutter spec that was heavily used at the beginning of TBC.
This was because originally, at lower resilience levels, blessed resilience was superior to focused will. What changed was that in Season 2, the available resilience and other factors increased priest survivability to make disc's mana efficiency and utility overtake the the advantage that blessed resilience had over focused will.