Probably a range of factors is my entirely uneducated guess. As noted by a friend of mine, the jacket isn't scored like modern defensive JHPs. Higher velocity might have helped. I'm confident that it would have opened with no denim.
A while back I tested a 200 gr HAP over 8.3 gr of 800X. I didn't get any good video of it so I didn't post a video but here's the pic of the bullet:
It looks like the Norma bullet might have been heading in that direction if it had more velocity.