Most Dragon Dancers are Dragons, which means that either they use Dragon Claw, which is kinda weak, or Outrage, which locks them into it and confuses them once its done. Tyrantrum gets better STAB in Head Smash that is more powerful than Outrage and still allows it to switch moves if something that can actually take it comes in, which is nice since Head Smash / Dragon Claw / Earthquake, iirc, is perfect neutral coverage on everything except Bronzong. Apart from that, physical bulk and the ability to turn Talonflame into a liability are decent.
So what you're saying is that Tyrantrum's only niche against other DDancers is the fact that it doesn't need to use Outrage to do damage, in exchange for a much, much crappier defensive typing and slower base speed?
If we're looking at the facts, Haxorus, which is imo the most offensive-oriented DDancer, dropped in usage into UU. I can't see Tyrantrum faring much better considering they both cover the same niche(slow but powerful DDancer), what with weakness to 3 priorities. Tyrantrum is a beast in the offensive department, I'll give it that but it's defensive capabilities leaves a lot to be desired, imo enough for it to not make to OU. Just ask Kyurem-B, who got booted to OU because of it's crappy typing + barren movepool despite it's uber-worthy stats.
One more thing, OU just gained 2 DDancers that don't use Outrage to do damage. Yep, I'm referring to ZardX and Megados.
Also, nearly everything on your list of counters also applies to Terrakion and Tyranitar as well, and both of those are pretty decent, right?
Comparing Tyranitar to Tyrantrum is imo dumb because they cover different niches. Comparing Tyrantrum to Terrakion though, as a set-up sweeper yes they might have the same weakness BUT set-up sweeping is not everything that Terrakion can do though, hm? Not to mention better stat spread and Terrakion actually makes use of its dual typing to give it dual STABs.
Basically, those two you mentioned brings soooooo much more to the table that it offsets the crapiness that comes with the Rock typing, something Tyrantrum doesn't have.