Because the whole point of that kind of change is to give USF cooldown only barrage weapon that all other factions have in form of rocket arty(and brits with their gimmicky but now I suppose functional base howis).
UKF base howies aren't an indirect fire substitute with their current throw range. They're very slow, ineffective grenades.
Until the flares actually outrange HMGs, they don't serve the role of a mortar or a rocket arty, because you've got to pull off a flank to throw the flare down. If you've done that, you might as well just throw a normal grenade at the HMG because it's more effective.
The only real use of UKF base howitzers right now is stunning vehicles in team games.