I base it on the lack of top contenders or tier1 teams in CS 1.6 , Quake and CS:GO.
Korea has provided with WeMadeFox
China has provided with wNv.cn
Do not remember any other contender from the asia scene.

Also, overwatch is at an infant state and requires several fixes / tweaks to be come a solid esport. I think that as in every major FPS game , EU can overcome ASIA if it becomes no.1 esport.

Also , don't shoot me ...I am just stating my opinion :D

YES , agreed.
The FPS talent of koreans though is weak in comparison with the EU talent.

With the exception of SC:BW and maybe wc3 in its prime , koreans always become the best in the lower skill demanding games.

Also, if the overwatch league becomes NA only and there are no mechanisms that will enable new players to get 'attention' from orgs in other regions....Will lead to Overwatch dying competitive just like every Blizz game.