Well I mean they don't have the greatest movepool ever. Clefable has all it needs to be a great support mon: TWave, Wish, Softboiled, Heal Bell, Stealth Rock, Toxic etc. What does Landorus have? Stealth Rock. What does Garchomp have? Stealth Rock + recoil, and maybe Toxic. What does Heatran have? SR + Toxic. I just don't think the degree of support they give is as good as Clefable when it has more support options.
And that's not all. Out of all the mons you mentioned, none of them have reliable recovery. Garchomp will eventually die. Landorus will eventually die. Heatran will eventually die. Clefable? Just give it any of Moonlight, Softboiled, or Wish and it's set. It's able to stick around for much longer, and it's helped with immunity to weather, entry hazards, and two forms of status.
But wait, there's more! Clefable's movepool not only lets it support its team and stick around for longer to keep supporting, it can use these moves to BS past it's checks. Even though the pokemon you mentioned have "great" typing, they all get destroyed by common moves. Lando+Garchomp? Ice Beam, Icicle Crash, Scald, Hydro Pump, Ice Shard, Surf, Waterfall, Hyper Voice, Outrage, etc. Heatran? Any Water, Ground, or Fighting move will do. Clefable? It's only weak to two types, both of which are seldom used offensively except on Metagross, Scizor, and Gengar. And two of those pokemon are highly susceptible to TWave, a move which Clefable commonly carries. You can argue that Heatran has Power Herb Solar Beam and Landorus has Smack Down, but how well do those work? Landorus has to give up a powerful or utility move to run Smack Down, and usually is also forced to run SD for the wallbreaking power, and Heatran's Solar Beam is a one-time use. Clefable? Not only can it TWave its own threats multiple times, it can TWave threats to its teammates as well, all while healing itself up. A paralyzed Metagross won't be doing much anymore. Neither is a paralyzed Gengar. Neither is a paralyzed Charizard. Neither is a paralyzed Tornadus. Neither is a paralyzed Hoopa. Neither is a paralyzed Weavile.
I'm not saying your mons are bad at supporting. But I am saying that Clefable does it better with more support options and reliable recovery. This is what IMO sets it above the other support mons in OU, making it a cancerous S in my eyes.