Garchomp has become a jack of all trades but master of nothing, in a sense.
Practically everything it can do something else can do it better nowadays. Great Tusk and Ting Lu and Gliscor and Landorus for instance are better at being tanks or general utility like setting Spikes. Things like Baxcalibur are much better offensive threats.
It's once trollish speed is now too slow, Scale Shot helped to salvage this during Gen 8, but it was removed from its movepool until the Teal Mask DLC and by that time it was too late between new toy syndrome and Baxcalibur and Roaring Moon both getting it and being able to use it a lot better than Garchomp.
There's also been a ton of power creep in general, and new Pokemon like Iron Valiant and Enamorus put fear into it.
In short, it simply couldn't keep up. It's outclassed both offensive and defensively, new threats have come to be, it's not been given any new meaningful tools and has even gotten some removed permanently or temporarily, and last but not least GF still thinks we're in Gen 4 and so continues to refuse to give it Dragon Dance out of fear it'll make it broken.