It's about how easy it is to reach the € sign. Ideally, it should be as easy to type as the $ sign is in the en_US layout.

For what it's worth, I think most all European keyboard layouts have key combos for € and $ defined (many have £ as well), while on en_US you can only type $ (without messing with settings). Europe of course has more currencies than just €, but they use a two-letters-long abbreviations instead of a special symbol.

zł has entered the chat. ;-)

(The Polish Ł is typically not easily typable of non-Polish keyboards.)