It's left out because its bad lol. Let me list the reason you don't play Ninetails GX in a Greninja BREAK deck.Because your set up might not be complete and keeping your Froakie based line on the bench is the safest way to ensure there will eventually be a Greninja BREAK on it in the first place. We are talking BREAK stage 2 Pokemon here and I really don't know how Ninetales' Safeguard somehow is left out of the discussion here.
When I'm going into Alolan Ninetales I'm not only covering the GX side but also the regular one from BUS.
1. Its a GX in a deck that doesnt run GX's. The whole point to the deck is to win the 1 for 2 prize trade, this completly negates that.
2. Metal weakness when Metagross is one of the Top 3 decks right now and going into next format, meaning its a free kill.
3. Searchablility. Are you seroiusly going to dedicate your only search card in Ultra Ball to a Ninetails and not your Frogadiers/Greninja?
4. Too long to make use of it. It has two energy cost to do anything. and the deck doesn't run much free energy to begin with. All of your energy goes to Greninja for Water Shurikin.
5. No one, and I mean NO ONE with a brain, would ever Water Shuriken, then retreat into something that can't do anything. If you have Greninja BREAK on board, you have a Water energy to attack with. So you will attack with Greninja.
6. Deck Space. What sounds better? Playing 1-1 Ninetails GX for a "big HP wall", or playing 1-1 Starmie for Energy Retrieval/1-1 Octillary for the Draw Power.
7. Bad tech. Even if you are talking about the little Alolan Ninetails, you are still playing a 1-1 line in a deck that is designed to beat the big EX/GX decks. You send that up, and its going to get bypassed by Guzma. If it doesn't, how do you plan on getting out of active? The only good way would be to Guzma it, but are you going to waste a Guzma as a switch card? If you retreat, then Greninja is stuck as active and Ninetails is useless. If you play float stone, thats deck space that you're clogging up. It's a bad tech.
Playing either one of the Ninetails in Greninja is just a bad idea. The deck works better without them, and there's better, more useful techs to play.
As before, if your not willing to try it, explain why you deem it to be bad? Is it because it does not assist in the set up? (Because it clearly does) Or is it because you don't feel you need time to complete the Greninja BREAK evolution line? (Because you do). Alolan Vulpix assists in the set up, Alolan Ninetales gives you the time...
It doesn't do anything for the set up. It doesn't draw, search, or give you any kind of card advantage. Vulpix does, maybe, but if you go first, you want your first attack to be Frogadier, and if you go second, MAYBE you can justify the Vulpix, but unless you play a lot of it, you aren't getting any value out of it. And if you go second, its almost 100% better to play Talonflame anyways. So where does it help the set-up? And Yeah, you need time, but let me waste that time but trying to get out the clunky Ninetails line, rather than getting my Greninja line going.