Because Alex's best response to her running is to not run.

因为亚历克斯对她参选的最佳对策,是不参选

耶鲁公开课 - 博弈论课程节选

So if Player I chooses S1*, Player II will want to choose S2* since that's her best response.

所以当参与人I选择S1*时,参与人II就会选择最佳对策S2

耶鲁公开课 - 博弈论课程节选