Because of the way CASE works from the original BattleTech rules. Let me explain:
Ammo explosions in the tabletop games were very deadly things to the victim. If your ammo exploded either because of a critical hit by enemy fire or heat, your mech took massive damage because of the way the rules worked, and this usually resulted in your mech dying right then and there.
Let me use an example: Machine Gun ammo. One ton of ammo is 2000 shots. They way the rules worked, if that ton of ammo exploded, YOUR MECH TOOK 4000 DAMAGE! And the armor values from table top are only half of the numbers you see in MWO. So if that ton of ammo was mounted in a side torso, the side torso would be gone and then the rest of the damage would "bleed over" to the center torso and wipe it out.
CASE came along in a rules update, but the way it worked is that it contained the explosion to just one body part. So using the example above, if the machine gun ammo exploded, the side torso would be gone but none of it would "bleed over" to the center torso. It never prevented the ammo explosion in the first place.
I have no idea why CASE is mounted into the side torso of an Archer. Chalk it up to bad mech design.
Anyhow, the rules for ammo explosions in MWO are less severe. And this is one change I'm glad the developers made.
Edited by Signal27, 30 July 2016 - 07:01 PM.