I'm assuming you're talking about National Pokedex, National Pokedex AG, National Pokedex UU, or National Pokedex BH, because I'm pretty sure those are the only 2 gen 8 singles formats where those 3 Pokemon are allowed.
National Pokedex: Incarnate Tornadus and therian Landorus are more popular because therian Tornadus and incarnate Landorus are banned. Both forms of Thundurus are unpopular, and which one is slightly more popular randomly changes every month.
AG and BH: All 6 forms are very unpopular.
UU: Of the 6 forms, incarnate Tornadus is the only one that's allowed.
So Landorus in standard National Pokedex is the only therian form that's more popular than its incarnate form.