This is pure speculation. But, the other proprietary connections are ones where the engineer can design the connections, but they must purchase specific hardware from the vendor to construct it. That way, the vendor automatically gets their cut.
Side plate connections, I believe, are different. In order for that company to make any money off of their proprietary connnection you have to go through them for design. Therefore, it would not make sense for them to publish a standardized design procedure in AISC 358. If they did, then how would they ever make money off of these connections?