If I had to guess at the thinking of a person I've never met...
"Ok, if this hits it will likely take the Wizard to zero. These players are new, the enemies would have reason to capture them, so I'll have them just knock them out with no death saves involved, that won't be too punitive annnnd I crit and killed him."
That kind of scenario. I can get it. I don't like killing players. They have to really screw up in the face of information they knew for me to feel it was "deserved". But at the same time it is cutting out some spice from encounters. I'll be considering whether this is something I keep, unlike say, automatic success/fails from 20's/1's, which I know I won't use.