The reason why NA teams are falling behind Asian ones is simply because Asia is the hotspot for E-sports, and they're culture encourages it much more than ours does.
If you grew up in Renaissance/Baroque period, you would be a much better musician if you were born in Vienna, than if you were born somewhere else.
Location and culture plays a huge influence on your craft.
Not only that, but since Asia encourages E-sports, now they have a MUCH larger pool of professional gamer players to choose from.
E-sports is a business in Asia.
E-sports is a hobby in U.S.
And... well... Most of the NA teams, if not all of them are cocky kids with huge egos, and overall terrible attitudes and/or work ethics.
If NA team practices 6 hours a day, A korean team will practice 10 hours a day, with 2 more hours of analyzation on their practice.
I remember Locodoco talking about Azubu Frosts schedule, it was like 5 hours of solo queue, 5 hours of scrims, you had to watch all of the days replays, and if you didn't, or you were late to a scrim, etc. You had to do all of the house chores or something. If you underperform, you are able to being kicked/benched for under performance. They also aren't allowed to have girlfriend/boyfriends lol.
NA teams aren't practicing to be the best they can be, because they are making plenty of money off of streams. Being a popular streamer > being best team, because streaming is where the real money is at. (Streaming is also a MUCH more stable income)
Basically, Asian teams live and breathe their current game. While NA teams doze off playing tetris, street fighter, D3, etc. And when they do play, they usually rage at their team in solo queue or make some other stupid excuse as to why they are not owning instead of trying to improve their gameplay, or they rage at each other/troll their team in scrims.