The fact they win everything and the NZRU loves them. Also, the amount of depth they have beyond the 20 All Blacks means that they have no weak teams, in comparison to say Otago who's first team is probably on a par than the Crusader's backup.
They take players from throughout the rest of the country and then claim them as their own. A mate and I did a survey of Crusaders players a couple of years back and (as I recall) we found that only 8 out of the wider training group were born and bred Cantabrians.