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.
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.