Porygon-Z was deemed uncompetitive; look at SDS's post
here. I feel like explaining myself as well, though, before moving this thread to L&O.
I talked this over with Seven Deadly Sins, Iconic, Eo Ut Mortus, and PK Gaming, and it was agreed on that Porygon-Z isn't viable in BW OU. Choice sets are ridiculously easy to counter; tons of things can switch-in on Tri Attack, such as Tyranitar, Jellicent, Ferrothorn, Terakion, Blissey, Chansey, and so on. All of these Pokemon are common and can easily take advatnage of a Porygon-Z locked into a single move. Agility sets are too easily revenge killed by Roobushin and Scizor. Porygon-Z can run Nasty Plot, but why would I use it when I could use other / better Nasty Plot users such as:
Thundurus
Has Mischievous Heart, is much faster, is resistant to Mach Punch and Bullet Punch, meaning it isn't revenge killed as easily as Porygon-Z is.
Mew
Also has Nasty Plot and is significantly more bulky than Porygon-Z / has better typing. Unlike Porygon-Z, it has access to Flamethrower, allowing it to torch Steels. Porygon-Z has to rely on HP Fighting to beat Tyranitar, while Mew has Aura Sphere. Lastly, mew gains STAB on Psycho Shock unlike Porygon-Z, making it easier for it bypass Blissey.
Celebi
Has more bulk and resistances, making it easier to set-up. Also has Giga Drain + Recover to keep it healthy (Porygon-Z has Recover, but it's too frail to use it and loses out on coverage).