It used to be, in the past, that this decision making on what race to get into was done in the backrooms, privately without much fanfare. Must be that the world of political campaigns has changed in which a person who wants to run for office needs to make their choice(s) public so that they can see where the support money will go to. 

Maybe it's forced transparency by the members of the parties, sick and tired of having their candidates dictated to them by faceless, nameless individual advisers who sit "behind the curtain".