Question

Why isn't Glastrier and Spectrier in Ubers?

Comments

They're not in Ubers because they are not good until they fuse with calyrex

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I think you may be confusing Glastrier and Spectrier with Calyrex-Ice and Calyrex-Shadow; those are in Ubers because they have much better movepools, stats, etc.

Answer 1

Because they aren’t strong enough.
Not all legends are in Ubers. Spectrier and Glastrier’s stats are far surpassed by every Pokémon in the Uber tier, as well as the fact that Spectrier’s movepool is atrocious. They just can’t compete with Pokémon in the Uber tier, so they aren’t there.

Comments

They have Neigh abilities that make them OP.

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They are no better than Moxie, which Mighteyana and Pyroar have, yet they are in low tiers.

Answer 2

Update
After spectrier ban, Spectrier is banned to ubers, but its not amazing there, outclassed by shadow rider calyrex.
Also I should add that glastrier is ice type so it has to use boots and its slow.