I think its for good reason, fact alone is that it can almost be threatened with a 1HKO by a Luvdisc of all things and that at the very best it only ever has 80% chance of connecting with its attacks. Its can be somewhat unreliable.
Blaziken is outsped and OHKOed by luvdisc in the rain, so obviously it should be RU. Oh wait...
Simisear outspeeds and OHKOS Ferrothorn, so Ferrothorn sucks. Alakazam gets OHKOed by Farfetch'd, so it sucks too. Unfortunately dude, it just doesn't work like that.
Regardless, Durant is great. Honestly, the only reason noone uses it is because he doesn't get EQ. Like, look at this:
+1 X-scissor Durant vs. 4/0 Victini: 98.2% - 100% So you can't switch him in.
+0 Iron Head Durant vs. 252/4 Hitmontop: 77% - 90% clean 2HKO, and only has to fear CC. Still 2HKOs 252/252+ variants, and they fail to 2HKO back.
Kyurem - Banned so whatever.
+0 Stone Edge Durant vs. 4/0 Arcanine: 100% Although this shouldn't really be considered.
+0 X-scissor Durant vs. 252/0 Donphan: 50.3% - 59.4% A clean 2HKO, while he can't kill since Durant outspeeds.
+1 X-scissor Durant vs. 4/252 Chansey: 75% - 89% I find this funny, but I think Chansey should be banned anyway so whatever D:
+0 Iron Head Durant vs. 4/0 Heracross: 94.4% - 100% while Heracross can't OHKO back unless it's guts boosted.
Zapdos: Walls it, but he's the only exception.
+0 Iron Head Durant vs. Nidoking: 92.1 - 100% This wouldn't go well if nido is scarfed, but otherwise he's destroyed if he switches into a hone claws.
Vulpix: Probably won't face anymore since drought's gone, but +0 iron head does 99-100.
you do realize that he destroys about 70% of the top ten, right? How does he suck in any way?