If I had to guess, I'd think they wanted knocking out to be a way to feel good and not forcing you to be a murder-hobo. Letting the NPC actually live to yabber another day seems reserved for special scenarios.
Well... there a a few instances I can think of where knocking someone out (instead of killing them) will have huge consequences later on. Not going to spoil the one that makes the most difference, but in the hag-fight for example, one of the OG masks (the black Halfling girl) will make another appearance during Act III and comment on the fact that the party spared her life.
Took me a few runs to even realize it was the same character as the game only has so many face-models/hair-styles (and voice actors) so I initially thought this was just a coincidence..
