The IS already 'got something' - they've had the most development time invested in them and have the most assets in the game.
They have the only hovercraft, the only VTOL, the only heavy fighter, two more mediums and two more assaults than the Clan. That's 35% more content asset-wise.
They have AECM, RACs, the Long Tom, seemingly weight-free ELRMs, Heavy Gauss, MRMs...
And their two tanks are arguably better than the Clan counterparts, why would they need a third at this point?