Stats alone don't make a mon great otherwise Regigigas would be top tier ou material
The same reason as why Mega Garchomp is considered worse than its regular form, its role, while it does an incredible job of doing, is still far too specific. Mega Garchomp is a powerful attacker and a bulky one at that but that's just it, it sucks at doing about anything else. Sand force is a horrible ability especially on a Pokemon whose attack already makes the rest of the tier's look like uu, not counting abilities or items
Landorus Therian on the other hand, is basically Batman's utility belt. It literally has every utility move in the game from rocks to knock off and even a switch move. Thank Satan it doesn't have reliable recovery. The bottomline is Lando can do a lot of things while being an incredible buffer against many physical attackers so that its teammates can get off their asses and pick up the slack
Basically, it's a matter of what roles they can do. Mega Garchomp has one role and that's it, to be an attacker or sometimes even pry apart entire teams with scale shot + swords dance. Natdex ou has no shortage of powerful attackers so not everyone is gonna want to use Mega Garchomp. Landorus Therian is not restricted to being an attacker. It can go for scarf, rocks, utility, whatever its team needs and as such, fits on a much wider of variety of teams, maybe even every non stall teams
These two can be made an analogy when compared to a swiss army knife, which is Landorus. While it's not the best at a specific job, it can do a lot of things. Mega Garchomp would be the equivalent at a kommando tri dagger. It's very good at one job, which is stabbing, but sucks at everything else
There might be some info I'm missing since I only started natdex a couple days ago but that's what I've noticed thus far. These mons barely changed from the gen seven days
