As long as we're talking about Dragonite, the only thing Salamence doesn't completely outclass it in is support. Then again, Salamence does get access to Wish somehow; if it could pick up Heal Bell from the move tutors, why, there'd be no reason to use Dragonite ever, because then you'd have Salamence being the best offensive Pokemon in OU and a great support Pokemon.
But that's not important; what's important is that Dragonite is so much easier to stop, even with its great defenses, just because so many things can outspeed it and lay on the hurt. Salamence already starts at base 100 speed, which outspeeds a lot of OU as it is.
Executive summary: Dragonite has good defenses because it needs them to take hits from faster threats. Salamence's defenses aren't that much worse, but what does Salamence care when it can KO pretty much anything as it switches in? (Also, Intimidate.)
But that's not important; what's important is that Dragonite is so much easier to stop, even with its great defenses, just because so many things can outspeed it and lay on the hurt. Salamence already starts at base 100 speed, which outspeeds a lot of OU as it is.
Executive summary: Dragonite has good defenses because it needs them to take hits from faster threats. Salamence's defenses aren't that much worse, but what does Salamence care when it can KO pretty much anything as it switches in? (Also, Intimidate.)