Ultimately, I think it all really comes down to the studio wanting to hire an A-list actress who'll get people into theatres, so naturally Johansson is one of the few young female actresses who'll foots that bill.
In comparison, how many female Asian actresses can boast of that level of popularity in the West? Basically none whatsoever.
It would be completely naive to believe that she was hired by the studio for nothing more than being the best actress in the role...and even more so to believe that an actress of Johansson's repute would even have to audition for the part in the first place. For the studio, it's simply a matter of making the most amount of money with this film as possible. As a result, with Johansson playing the lead, for the executives it guarantees the film a certain level of financial security it otherwise wouldn't have had with an Asian actress in the role.