That is the question that, I believe, only the Nobel organization can address. Without in-depth investigation, all we can do is speculate, and it requires no knowledge or skill in anything to do that.
Maybe we will see some female prize winner this year?
In 2017, the Noble Prize committees have made efforts to obtain nominations from more women by inviting more female researchers to nominate for the Nobel Prize 2018. According to the permanent secretary of the Royal Academy of Sciences.