Because on ubers Hazard's are kinda rare, and even then, Lugia is still having HUGE troubles to wall things, even Shedinja is somewhat viable as that metagame is almost fast paced as doubles. Dragonite can wall things, but it doesn't do that really well at all with Stealth Rock up, as when you're trying to wall, the opponent's just switch-in to it's favorite Ice Beam user, and it's the time to go for Dragonite (and if you don't roost the damage to recover your Multiscale, say goodbye as your counter will destroy you on one turn), the same goes for Salamence, that is OU only because it's offensive prowess; yes, it can wall, but if it were it's unique viable set, it could be already UU as a long time. Gyarados can't really wall, it just force switches and take advantage of it with it's offensive presense and ability.
I love Caterpie, he's a so adorable little bug! ♥ (Also, sorry for my English, I'm still learning it and I'm not a native speaker).