Question

What was the reason that Defog became so popular in gen 6?

Answer 1

It was buffed

Before Generation 6, Defog only cleared hazards on the enemy's side of the field (source) and the evasion drop didn't matter most of the time. This made it one of the worst moves. However, Generation 6 made it remove hazards on both sides of the field, during a time where spinning was hard because of Aegislash and Gengar, and because the only viable spinners were Starmie and Excadrill. Defog was also given to some already great Pokemon like Latios and Zapdos.

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Answer 2

Gen IV and V
Defog clears away any fog on the field, as well as Light Screen, Reflect, Safeguard, Mist, Spikes, Toxic Spikes, and Stealth Rock on the target's side of the field.

Gen 6 On
Defog now clears away Spikes, Toxic Spikes, Stealth Rock, and Sticky Web on both the user's and target's side of the field as well as Aurora Veil on the target's side of the field. It still only clears away Light Screen, Reflect, Safeguard, and Mist from the target's side of the field.

There was absolutely no reason to use Defog before Gen 6 because you were just helping your opponent clear hazards. With Gen 6, it jumped up in usage because it now cleared Hazards on your side (even though it cleared for your opponent as well) and it was a more reliable option that Rapid Spin (which only had a few reliable users and Ghost Types in the tier being apparent). Many viable Pokémon were also given it.
Source: https://m.bulbapedia.bulbagarden.net/wiki/Defog_(move)

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Thanks :)