A gauss rifle which can explode.
A gauss rifle which needs ammo.
A gauss rifle which cannot reach as far.
A gauss rifle that the Blood Spirits would rather put on a Mech with their limited resources.
A gauss rifle that does not match the general Blood Spirit logistics system.
Btw, that 13 tons your mocking? Give it better protection than the Partisan variants. A little less than the Fury. More than the Demolisher which totes two gauss rifles. More than the Schiltron Omni-veh the Dracs put out. Nearly double the old Schrek PPC Carrier. Same as the Alacorn Mk VI which has 15 tons on it, and is a ton under the Challenger X which also has 10 tons on it.
Granted, first upgrade it to make it FF armor but hey, we are talking about the Blood Spirits here.
IMO, it all depends on when your fluffing your Clan armor out as . . . post Invasion the Invading Clans started making some armored bricks like the Heimdall, Shoden, Enyo, Tyr, and more. The only improved armor designs we see from the Homeworlds would be the Spirits building the Morrigu, which makes sense due to their limited resources.