2016-2020 : BTS Worldwide growth --> Better Kpop album distribution globally
2020 : Pandemic
This deserves best answer. BTS's effect on the distribution of kpop albums in the west and on online platforms such as Amazon really changed the game. Several years back my friend who is a 2nd gen stan had to physically go downtown to the lone kpop store we had to buy physicals (she didn't like to ship from stores in Korea bc they took ages to arrive). But nowadays every store in America, online or in person, is selling albums so ease of access is the biggest reason for growth in sales during that period of time. And that access only came because of BTS's growth here.