Looking at the listings in Sarna, they have the original flavor Atlas, the Atlas II (a SLDF "royal" variant), and the Atlas III (a Jihad/Dark Age variant intended to fill a similar niche to the II). While many original Atlas pilots stayed in the IS, the AII's pilots chiefly followed Kerensky with their machines, which in turn were treated as valuable relics by the later forming Clans, with at least one even making it all the way to Tukayyid during the invasion.

Given that it was both a solid design and had that golden association with Kerensky himself, why didn't it get the "IIc" treatment with so many other Star League designs that were brought along on the Exodus? Was it just a meta decision because the Atlas was a signature IS assault mech, or was there an in-universe reason?