As I commented, Spectrier has a terrible movepool
It’s best STAB move is Shadow Ball and it’s best moves are probably Dark Pulse and Shadow Ball, which is easily walled by Fighting Types and Normal types
On the other hand, Glastrier has a better movepool, consisting of Outrage, Icicle Spear, Megahorn, Stomping Tantrum and Superpower
Glastrier is also used with Trick Room
With you outspending everything, you have a better chance to knock out your opponents Pokémon with ease, getting boosts from Chilling Neigh
With access to Swords Dance as well, its attack will be monstrous
With its boosted attack and insane bulk, once Trick Room runs out, you can either set it up again or just wall Attacks and then hitting like a truck
As Xky1 has said, with broken dragon types like Dracovish and other dragons that are in SWSH, you need the Ice Type to OHKO them fast
Hence, Glastrier is probably used over Spectrier with the reasons I have stated
Hope I helped