Pikaboo is the best rogue on US but he hardly ques ladder. Whenever he's played seriously he's been high rated. However that is just going off topic.
On Topic: Human Rogues are really strong. More rogues on EU cuz everyone is Ally. Also everyone being ally on EU is not only just good for the fact that Human=More damage but also means they don't have to fight orcs. US rogues constantly have to fight orc warriors/shamans/hunters/etc etc, while also not being human themselves therefore they can't get as much pressure as a rogue on EU.
The same thing applies to why there are more Mistweavers on US/Less Windwalkers vs Less MW's on EU/More WW's on EU. Mistweavers need to be undead to be as strong as possible, along with them having a dps sham on their team making them unferable. WW's are similar to rogues in that no Orcs=Good+Human Damage=crazy good.
I feel people don't really take into consideration how much the factions between regions effect what classes/specs will see more representation.