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.