Suggestion: update the Papal Succesion page. Information is missing.
Missing information: the College of Cardinals mechanics requires you to be of the same religion as the College (Catholic or Fraticceli if you did that), you or your liege can't have an antipope, you have to have at least one theocratic vassal of any tier (all of these 3 are stated on the wiki) AND you also necessarily have to have diplomatic range to the Pope's realm.
I can completely guarantee that if you do not have diplomatic range, your priests will not show up as elegible to become cardinals. Proof: start a game on 769 as the Duke of Kent. Set diplomatic range game option to halved. Diplomatic range = 700. Province of Orbetello is within the Pope's realm. It has the same religion as you, so diplomatic range 700+100 = 800. Game option sets diplomatic range to halved = 800/2 = 400. Orbetello is at a distance of 424. Your priests will not show up on the College of Cardinals screen. Second experiment: start on 769 as the Count of Boulogne and Guines. Diplomatic range from Boulogne to Rome is 433, out of range, but Rome doesn't matter because Orbetello is within range at 383 range and the Pope's realm contains Orbetello. Your priests will show up as elegible to be Cardinals.
Playing in the British Isles with diplomatic range halved hoping to control the Papacy is not viable unless the Pope expands north to at least Ravenna (de-jure part of the Papacy) or you conquer a coastal province between Mortain in Normandy and Kennemerland in West Frisia.
Edit: I've recently been told that I might be partially wrong, and that in reality it is the priest himself that has to be within the Pope's diplomatic range. In essence, this person told me that if you play the count of Boulogne and Guines as stated above, and you conquered Kent, your priests from Kent would not be elegible as Cardinals. I have no time to prove this theory, but it should be as easy as starting a game in Stamford Bridge as William the Conqueror with halved diplomatic range and checking if only your priests in Normandy are elegible. If some dude anywhere in Britain is elegible, and the Pope doesn't have provinces closer to Britain than Orbetello; then it's only your diplomatic range that matters, and not your priests'.