It makes no sense for a political party to pursue a policy that will alienate the state's voters. In each case, a handful of people makes up the party leadership. Republican and Democratic precinct officers elect representatives to their parties' state committees.
Libertarians apparently choose their leadership in an annual state convention.
The people who run the parties are generally not the people who run for office under the party labels. Gov. Gary Locke found that out when he urged his Democratic Party leadership to drop its lawsuit against the state's blanket primary system. Despite the fact that he is the most popular Democrat in Washington, his own party leaders are ignoring him. Despite pleas from state officials elected as both Republicans and Democrats, their party leaders continue to push for changing an incredibly popular primary system. Washington's primary is different from just about every other state's.