The reason Garchomp was so popular in VGC 2014 for instance was its great Spread moves along with a Solid Attack and Speed. Let's start with stats. Speed is the most important factor here. With 102 base speed, it was faster than many threats such as Mega Kangaskhan and Mega Charizard Y for instance. Also, it has the Speed and typing to counter other threats such as Rotom-H, Aeigislash, and Chandelure for example. Also, its coverage moves counter the extremely common Mega-Charizard Y and Talonflame, OHKOing them. Unfortunately, Talonflame does get to get off a Tailwind. Even though it has counters like you stated, it's good Speed and Attack Stats along with great STAB moves in Earthquake, Dragon Claw, and Rock Slide give it plenty of coverage against most of the Meta. In this case, the pros greatly outweigh the cons.
Forgot OU. The reasons here are mostly the same. There are a few additions though. It's offensive typing again forces many switches allowing it to get off a Swords Dance. This combined with a usable Speed, attack, and Life Orb make it a potent Sweeper. Even though it suffers with Mega Manetric and HP Ice, its pros again outweigh the cons. See (1) for more info.
Source: https://www.smogon.com/dex/xy/pokemon/garchomp/vgc14/
(1): https://www.smogon.com/dex/xy/pokemon/garchomp/ou/
Also, Smogon agrees Garchomp is a S tier Pokémon.
