I wouldn't necessarily say the the CAS that D* uses is more secure from a technology standpoint.
But what D* has done is put the fear of God into the people that would have the skills to pull off a hack.
A bit of research will show that when D* was being hacked by every Tom, Dick, and Harry that D* went on a legal parade. Even suing people who bought simple ISO7816 interfaces. The same type of device used to interact with a AmEx Blue card.
Basically it is the fear of the legal sh*t storm that keeps D* from being hacked publicly.
Yes, it was in-house development what kicked off NDS;
Dish tied their hands when created NagraStar LLC with Kudelski. Now they cannot change the 'horse'.