[pmjordan]

The long tenure (and prestige of the organisation) probably also contribute to the willingness of candidates to subject themselves to such a process. Getting someone who is currently employed to sacrifice multiple days of (paid or unpaid) holiday for your hiring process is presumptuous, particularly if they're actively looking for a new position and thus probably going through this with multiple companies simultaneously.

As you say, it's all about making the relevance of the process obvious to the candidate. Also, companies should be as accomodating as possible in terms of timing of appointments, etc.