Couple reasons. Just my opinion, not official reasons.
1. Because the 3rd Succession War has long been detailed what was in it, and Rifle Cannons weren't.
2. Even the Periphery was still making autocannons, they didn't need Rifle Cannons.
Some things were lost due to advanced technology. But some things were lost because they were old and just hanging around, and when the destruction of the early Succession Wars hit they weren't replaced with the ancient technology but with current technology.