From Diablo Wiki
This build was considered useless in the early days of D2C, until the discovery that (thanks to a bug) the Concentration aura boosted the damage of the hammers. Hammerdins then became popular, as players figured out how to play them (herd a lot of monsters into a large open area).
Hammerdins vanished from the realms in the early days of D2X, when the damage from Concentration through Blessed Hammer was nerfed, before reappearing, stronger than ever, in v1.10 when synergies boosted the potential hammer damage to enormous levels, and some of the new Runewords had stats that were perfect for a hammerdin's needs.
 Build Guide