The reason they're not in there is the exact reason we put the mortars up to 200k (now down to 100k). Tanks that use long-range MLRS systems are incredibly overpowered on a small island such as Stratis, and could cause devastation to the opposing team if you program the artillery computer to strike the right area. On all of those vehicles, from the artillery computer, you are able to strike the entire island from one place (and way beyond, for that matter.) Park it next to an ammo truck and you've got the full set. Another thing to factor in is the size of the rounds. 230mm of freedom (sandstorm) is what you'd see taking down whole building blocks - to put that into perspective, the mortar uses 82mm rounds.
If they were implemented, I'd estimate the price to be upwards of 200k, and/or perhaps disabling the artillery computer.