In short, participants must register for each event.  This is due to the fact that events are distinct and often have differing requirements and new fees.

That said, participants in previous events can click the Autofill link above the form where they are asked to set a login name.  Clicking that link and logging in with existing credentials allows the participant to use the same login as in the previous event.  Their registration form will also autofill with biographical data such as Name and Email address.