Landorus-Incarnate is in Ubers because pairing Sheer Force with Life Orb removes the recoil (on moves with secondary effects.) It has a STAB Earth Power that cleaves through lots of defensive Pokemon. It slams Toxapex, the most common defensive mon. Ferrothorn has a great chance to be knocked out by 2 Life Orb Sheer Force Earth Powers after Stealth Rock. However, clever players can predict a Ferrothorn switch in, and if they do, Focus Blast will either OHKO it after Rocks, or it will put it in Earth Power range. Even the mighty Blissey wouldn't like to face a Focus Blast. Forget about Clefable, Sludge Wave has more than a 30% chance to OHKO. Notice how I italicized defensive, these are your best bets for defense, and they crumble to Landorus-I. The Crown Tundra OU metagame is much more offensive, which means more prey for Landorus-I. This lack of safe switch-ins is the main reason why it was banned from OU, putting immense strain on teambuilding.
Landorus-Therian is the most popular Pokemon in Gen 8 OU, and that doesn't seem to change anytime soon. However, it isn't Uber material. It has an average Speed stat for an offensive Pokemon, while Landorus-I has good Speed. A max 309 Speed means Landorus-T is outsped by Tapu Lele, Spectrier, +2 Magearna, Garchomp, Hydreigon, Barraskewda, Swift Swim Kingdra, and some other things. Choice Scarf is an option, however it means Landorus-T can't use Swords Dance to break past walls. It also means it can't use Stealth Rock or Defog very effectively. It also has to work for KOs, as Ferrothorn, Clefable, Corviknight, and a few others wall it or can switch in.