Different roles I'd think. The IS designs Pocket WarShips in order to replace WarShips, and to fight WarShips, so they need the longer-range guns in order to fight WarShips at similar ranges.
Clan DropShips however, are designed to escort their WarShips. They don't have many, but they do still have some, and the Clans need their Pocket WarShips to deal with things like fighter swarms and small craft. They can rely on their WarShips to fight other WarShips and larger DropShips. What they need are escorts, especially the older Star League designs, that can deal with the smaller foes.
Though admittedly, this might leave the Clans rather stretched out in a Naval capacity. Their WarShips fleets aren't that big. It could be interesting to see how Clan and IS DropShip fleets match up without WarShips.