Most of Affliction warlock's strength in Mythic+ is due to the prevalence in power per
Soul Shards utility. Before, you would use a single
Soul Shards to cast a single
Unstable Affliction on one target. Not very impactful, except for focused single target damage. Your only real AoE choice was to spam
Seed of Corruption.
Now, with the addition of
Malefic Rapture, you now have a strong nuke that hits every single target affected by your DoTs. To break down what a quick AoE pull could look like for an Affliction Warlock now, with no major cooldowns, you can throw a few
Agonys out, cast one
Seed of Corruption to apply
Corruption to everything, cast
Vile Taint, then spam
Malefic Rapture until you run out of shards. While this may not be the most ridiculous AoE output of any class (see Destruction Warlocks), it's essentially available for every single pull. In addition to this, Affliction is typically known to be one of the best scaling specs in the game - with endgame level secondary stats, it's no surprise that a spammable AoE nuke has nudged Affliction into the limelight.