Technological progress, the Wars of Reaving, the Jihad, evolving character, better homegrown designs, the IS Clans settling into their now-permanent homes, faction variety—all likely played a role phasing out the Warhawk from a factory line-up. Take your pick. They all fit to some degree.
Since there are plenty of individual BattleMech designs that survive 500 years of continual use, there's nothing stopping a Warhawk from showing up on a gaming table or say, the Brotherhood of Randis.