EVO, despite being a heavily US based tournament has become more and more international over the years?
Evo2009 -> 2 countries in top 8
Evo2010 -> 4 countries in top 8
Evo2011 -> 3 or 4 countries in top 8 (Latif played for US and Saudi Arabia.)
Evo2012 -> 6! Countries in top 8
Evo2013 -> 5 Countries in top 8
It's not like US is slipping, the rest of the world is stepping up and fielding representatives from around the world.
Twice Daigo was the only japanese player in top 8 (and in 2012 it was quite an upset since it was a year with one of the largest number of Japanese representatives ever.)
The real trend is:
Evo 2009 through 2013
Zero top 8 appearances by Euro players.