That doesn't explain Washington State, where the Democrats didn't use their state-provided primary at all (all of their primary delegates were allocated by a caucus you had to attend) and the Republicans did (though only one candidate is running now, so it doesn't really matter).

The Democrats made you physically get up on a Saturday and go, unless you had a valid excuse. Elections in Washington are by mail, so to vote in the Republican primary, you only had to walk to your mailbox.