Celesteela is very outclassed in this metagame in nearly every scenario. It is outclassed defensively by Corviknight while it is outclassed offensively by nearly every breaker.
This essentially covers it. However, I will explain why it is outclassed.
It has an over reliability on Leech Seed and Leftovers for Recovery (Combined with its great typing leaves something to be desired) while Corviknight has Roost for reliable Recovery. Additionally, it lacks any Utility outside of Leech Seed while Corvknight gets Defog or Taunt (which it runs on some sets). This is why Celesteela is outclassed Defensively. Lacks reliable Recovery as well as any Utility. It is forced to run Heavy Slam and Flamethrower to hit Mons like Kartana, Rillaboom, Ferrothorn, Buzzwole, among others to not get set up on. It then gets forced out by other threats such as Heatran and Cinderace for example.
Celesteela isn't completely outclassed, it is just hard to fit on just any team, leading to its lower usage rate. It can effectively run a Weakness Policy and Automize set or a Power Herb Meteor Beam set. Unfortunately, it has a lack of Coverage moves that still leave it to be forced out by a Variety of things, specifically Heatran, Zapdos, Blissey and even Toxapex, leading it to be having to be switched out and losing its boosts. This is true in any Format, but especially OU. All of these factors combined with the fact it's hard to fit onto any team, lead to lower usage. It also lacks any role at all outside of an Automize user.