I think Articuno's main problem is that outside its main STAB's is Psychic/Psyshock/Freezing Glare and Hurricane/Air Slash, it only has Shadow Ball, Hyper Voice and Ancient Power for coverage. Additionally, it has a fairly poor typing in Psychic/Flying leaving it weak to common threats such as Dragapult, Gengar, among others and getting outsped by all of them as well. It also has lack of coverage against common Steel types such as Ferrothorn. So, Articuno falls back from its lack of coverage, fairly bad typing, and Awkward Speed tier.
Hope this helps!