I think the logic comes from the fact Team Up is made from Japan’s SM9 Tag Bolt in the C block, and SM8a Dark Order in the B block. Since we’ve adopted the block system now as well, it would be weird to have very important cards that stick around for us when Japan rotates the B cards Jirachi or Koko Prism, but it would equally be awkward to rotate Pikarom or GengarMimikyu.