Coaches hire assistants, assistants are usually people they know especially at lower levels. Canadian junior hockey is an obvious stepping stone. USA is producing a better system at the junior level and will see the fruits of that labor.
For Euros, uprooting your life in your 30s to pursue coaching overseas isn't that easy - that leads to euro coaches unable to get past the original gatekeep. Then you have the uncertainty from GMs putting their job on the line to hire someone they aren't so familiar with to coach a style that may not translate from the differences in the game from Europe to NA.
Hopefully we can get more diversity (ha, I mean in terms of hockey thought) in the coaching pool by getting some ideas from the European pool.