Scizor may be very good at what he does, but his odd coverage, putting aside the fact that he's usually Choice-locked, means a myriad of dangerous threats can set up on him. Such threats include Keldeo, Volcarona, Forretress, Landorus-T, Thundurus-T, Tentacruel, Skarmory....even Heatran, Magnezone and Lucario can take advantage of Choiced Bullet Punch. All of these threats can become very dangerous after just a single turn of setup or a free turn in general, and Scizor can't keep U-Turn spamming forever because there will be times you'll go for Bullet Punch/Pursuit, the main reason you chose Scizor in the first place. Scizor is still fantastic, but his tendency to grant free setup turns for common opponents keeps him out of S imo. P.S. Usage does not reflect viability in the direct sense.