Because U-Turn Flygon isn't devestating.
I'm one of those people who won't ever give up on Flygon.
Sure he doesn't hit like a truck, but he's still probably one of the best scouts available. Personally, I hate revealed teams, and when you're playing without that it makes scouting much more useful. That gives Flygon his own niche again, basically where he was in Gen IV.
There's a whole list of other Pokemon that can scout, like Kojondo, the Genies (especially Landlos), and etc. None of them really have EVERYTHING Flygon has to offer. Scarfgon has plenty of resistances, and two very useful immunities, immune to Spikes and Stealth Rock resistant, outspeeds any unboosted pogey with the exception being like Deoxys-S, great dual STAB and enough other options to give great coverage.
It's not as bulky as something like Landlos, but it has dragon stab, and being a dragon gives him fire resistance, and water neutrality which can offer a few more spots to switch in safely. It doesn't hit as hard as other options, but firing off STAB Earthquakes/Outrages still pack enough of a punch that will allow him to pick off weaker pokemon reliably. He's not as fast, but he still gets the job done and has enough to offer over other options that it can be worth it.