Given the nature of random players in DF pugs, there's a plethora of different possible reasons for why you didn't get a commendation on any particular run. In the end, they're an incentive to stay civil both socially and in handling your role in the party, and that's only to increase your chance of receiving one, not some guaranteed perk you're earning by nailing your role perfectly.

I, for one, tend to focus on my role so much that 90% of the intricacies of how any of the other party members are performing are completely lost on me, the remaining 10% more or less being the bottom line (did the tank lose hate a lot, did the healer struggle keeping us alive, did it take forever and a day for enemies' HP to drop, etc). In the end, a player's conversation in the party chat is going to be far more easy for me to notice/remember at the end of the duty than how perfect their rotation and anticipatory responses were. I'm not saying no one can focus on their own role and still see the intricacies of others, but again, you're in a DF with randoms pulled from a large and diverse group, some of which are as incapable at focusing on their own role AND everyone else's as me, some of which don't even care enough to give a commendation. Even just luck plays a part.