They can't reasonably make Frozen Heart, Spirit Visage, or Warmog's options because they're the only tank items with over 100% gold efficiency (not counting passives). So adding bonus stats on top would make them way too strong for a single slot, especially when he can do that to two item slots.
Gargoyle's, Adaptive, Randuin's, and Thornmail are sub-100%, but they all have unique defensive passives that in the right scenario make them stronger than the items without defensive passives. So they fall into the same problem as the items that just have high efficiency from stats alone.
It's more reasonable for Ornn to turn 80% efficiency items into 100% efficiency items than to turn 100% efficiency items into 120%. Especially since they're now free and two of his items will transform the instant he hits 13.
Deadman's Plate is the only remaining tank item they could add, since it's identical to Sunfire in cost efficiency.
And to clarify, the reason it's fine for him to have so many ally item upgrades that are already above 100% efficiency is that he has no control over what they buy, can't choose which item gets upgraded, and doesn't grant them until late game, and most importantly can only transform one per ally, one ally per level after 13.
