Well, first of all I think is important to know it ISN'T A REMAKE of the (wonderful xD) japanese movie, is a film adaptation of the six first volumes of the manga (three per movie).
About the names, again, it's an adaptation that sets on Neo-Manhattan, a city rebuilt with japanese money (maybe that's why it keeps the name Akira, as there may be other things with japanese names).
For the third time: adaptation xD
I think people don't remember this enough.
Why DiCaprio? Because DiCaprio pull some stringes on the project and is a succesfell hollywood actor.
Why Manhattan? Who knows, on IMDB they say that New York was the unluckiest city ever xD
And I gotta tell you that DiCaprio with black hair is really hot xDDD
(Sorry if sometimes I don't make myself clear... english is my second language)