2002 CF: 2004 from Paris: 2006 VF: 2007 CF: 2009 Aragones, Ingles, Finisterre: 2011 X 2 on CF: 2013 'Caracoles': 2014 CF and Ingles 'Caracoles":2015 Logrono-Burgos (Hospitalero San Anton): 2016 La Douay to Aosta/San Gimignano to Rome:
Out of convenience, probably for the foreigners. The term 'passport' is easier to understand instead of 'credencial' (or credential) but of course we can't have these people thinking of showing their national passports at the albergue -- those would be filled with sellos in no time. Historically, the credencial was issued in one's own parish, as a sign to all people concerned on the route that the person carrying it is undertaking a pilgrimage.
define your terms! "Pilgrim's passport" is sometimes used as an English term for the credencial, although 'credentials' would be the straight translation. The CSJ calls their version "Pilgrim Record" specifically to distinguish it from the credencial. As marktqm says, this is a distant descendant of the credentials (= letter of introduction) carried by a medieval pilgrim as identity papers. Of course, today's credencial, whatever you call it, isn't really 'credentials' as it has no authority behind it. Incidentally, one of the first things the Société des Amis de St Jacques did when they were founded in 1950 was to issue a letter of credentials to prospective pilgrims.
Originally a passport was a permit, literally to pass through the port or entry point to a city or territory. So today's national passport, as identity papers, is descended more from the credentials than from the passport which is more like a visa.