Someone already answered this question but I'm gonna post my own because the current best answer is not the reason why Landorus Therian is so popular. No, the first reason why it is used that much is because of its versatility and role compression. Access to you turn, defog, knock off and stealth rock all in one Pokemon is a highly desirable trait as it can save you a slot on your team
The second reason reason is its typing. Being immune to two of the most common offensive types in the game is another highly desirable trait. Also, I believe it has the strongest unboosted stab earthquake in ou
The third reason is its ability. There is a reason why intimidate has been called as the best ability in the game. Despite Landorus' pretty meh defense and hp stat, lowering your opponent's attack stat gives it a deceptive amount of physical bulk. This is why it sometimes run bulky sets. Speaking of intimidate, that one turn where you lowered your opponent's attack stat could mean the difference between winning and losing even if Landorus has to be sacrificed just to activate intimidate. Pokemon is a game of inches and that one intimidate could be the deciding factor if you're gonna be swept by a physical sweeper or if you'll barely be able to hang on and eliminate it
With all of these qualities, Landorus Therian is just that easy to put on a team and it doesn't require that much support. The only ones I can think of that can pull off a similar level of versatility is Garchomp, Magearna and Tyranitar. It is true that Landorus just about instantly dies to any non physical super effective move but that is not its job. Handling special attacks is way above Landorus' pay grade even if they are neutral or resisted as they will slowly add up. That's why you're playing with six Pokemon and not just one
Source: playing against a Landorus Therian using player just about every other game
