They're meant to be very rare. I've given 3 in the last 4 years. 2 of those 3 times, the fighter didn't answer the bell for the next round and retired on the stool, so highly warranted.
new rules imply that it's based on duration, dominance, and damage. If 2 of the 3 come into play, it should be considered. If all 3, it should be a given.
How did you score the fight? Looked like Diaz took rounds 3 and 5. Even with the knockdowns it never appeared like Diaz was ever really stunned or else McGregor would have pursued.