I'm not exactly sure which Tangrowth we are talking about either. Funny story really, when Tangrowth Lv.X was first released last year, everyone had the same thoughts as you, Jackandellie. Then, Cities happened, and everyone realized that it wasn't good. Yes, once it is set up it can get good, but it can be easily countered, to be completely frank. Gengar leveling you down consistently ruins you, fainting spell ruins you, Luxchomp/BLG's speed is faster and they can possibly disrupt you before you can get set up, Blaziken kills you, Dialga can swing a lot bigger than you can, Gyarados can hit equally as hard turns faster(due to lack of energy attachment), something like a Tyranitar can get swinging and just pummel you with the same force, Sablelock just won't let you get a Tangrowth out, Lostgar will be faster and get stuff out of your reach, as well as leveling you down, and the fact that Tangrowth is slow, takes many energy, and needs two Lv.X to get its "tank" going. Getting two Lv.X, one of them being on bench isn't the easiest thing. On paper it looks great, but on the field it may not.
I would love for you to prove me wrong. I would love for Tangrowth, a rogue deck, to top or even win states. But the fact that nothing has changed in the past year for it(except maybe...the loss of Claydol and Roseannes?), as well as the fact that it is slow, and was sub-par to mediocre back then, as well as all of the counters listed above(just a few, a smart player can just outplay it), make using Tangrowth a very, very steep uphill battle. I'm not trying to be mean here, but have you, and only you, built, successfully tested and hammered out a functioning Tangrowth list, and then actually, legitimately, rigorously tested it against not only SP, but Lostgar, Gyarados, and Vilegar(for example)? Because testing against your friends dark type fun deck doesn't count as actual testing, when it comes to legitimate match ups.