Wikipedia says that it was founded by an American, Joe Coulombe, and then sold to trust which also owns a notoriously cheap and low quality German supermarket chain in 1979. Perhaps Trader Joe's started expanding more quickly after being sold.
But then, whilst i don't live anywhere near an Aldi, i have become something of a connoisseur of the baffling Teutonic delights of my local Lidl, so perhaps i'm biased.
But I have to admit I might be a bit elitist about this. Lidl, which I hear a lot more good about, also doesn't really meet my standards for a supermarket. And Albert Heijn, which is often considered pretty good, is not considered all that good by me.
Then again, I saw just recently a dirty one in Luxembourg, so my first comment should read "it depends" (which is kind of a non-comment) ;)