The main reason Io Shirai has decided to leave Stardom for WWE is simply because WWE is the place to be and she likely wants to duplicate Asuka’s success. From a financial standpoint, Shirai is going to be losing a lot of money as her salary in WWE will not only be less than in Stardom, but she will lose a lot of money in merchandise.
In Stardom, Shirai basically got to keep all the money she made on her merchandise, which was reportedly to be a huge amount. In WWE, as with all the other talent, she will only get to keep a specific percentage of cut from it.
In an interesting bit of backstage dynamics, Io Shirai and Asuka are reportedly not friends according to a recent edition of the Wrestling Observer Radio Program. Dave Meltzer stated that although the two wrestled in a tag team years back, they are not friends but that doesn’t mean that they are going to be fighting with each other backstage or anything of that sort.