at a guess its for market differentiation. like how I get the "international" version of US textbooks, which are exactly the same but with a different cover on the front. Or how Harry Potters and the Philosophers Stone needed a title change to sell to US. Or how you can get adult and child versions of the Harry Potter books.
Waldo always seemed a bit of a strange name, and it still confuses me why it would be changed for the US market. Anyone know why (Wiki doesn't say).