Mostly because it has slipped under the radar. If you look at the way the game has been until now. Something bigger than Salamence has always been around (Garchomp, Deoxys, Latias). Salamence is a huge threat, but those guys were bigger, so they got tested first.
Also, Salamence was only considerable for Uber after Pt, where Outrage came in. Before that it was more than manageable since it was in pretty much the same position as Gyarados. If Gyara suddenly got a 120 BP Water STAB move with 100% accuracy I'm pretty sure people would begin to question him as well (however ill-founded that might be, that isn't the point). I know a lot of people, myself included, who had reservations when we heard Mence was getting Outrage, because we saw the damage it was capable of after DD (Garchomp with SD is pretty much at the same level as DD LO Mence). The main difference is in survivability, but it is far from impossible for Mence to live for a fair while (Roost, when used well).
Course, it's all really about the whole "how much power is too much" thing. Salamence pretty much defines the borderline as it stands. One could (and frequently does) argue DNite is on the same plane, but the speed difference really really hurts, since DNite can't hope to even tie with the main "checks" for Salamence (Scarf Rachi and Flygon are the killers). Hell, even GYARADOS outspeeds it, and it's definitely not afraid of launching Ice Fangs or Stone Edges at it. That isn't to say Nite is useless (it's been the mainstay of my offensive force for almost all of my teams lately). Dragonite simply occupies different niches, and I feel it lacks the sense of pure terror that Mence brings.
(It could also be noted that Mence acts as an emergency check to Lucario, thanks to Intimidate - something DNite would KILL to have under most circumstances).