Things are too bulky for SD Sceptile in UU. Most random UU Pokemon survive a +2 Leaf Blade/supereffective nonstab move even though common Grass-types will be OHKOed by Acrobatics. Like Zapdos survives Rock Slide, Mew survives everything, etc.
This is why Sceptile's niche lies in RU, where all it needs is plus 2 and to get rid of that grass/flying gem and then it's pretty much almost guaranteed a sweep. Leaf Blade/Acrobatics/Earthquake pretty much kills the tier.