That's a really interesting question, Pach. I can see no reason to criticize harmless, decent, kindly stupid people other than bolstering one's (obviously weak) self-esteem. That's if you don't have any real connection with the stupid person involved. If you and the less gifted person are in some way forced into the same group (family, school, work, marriage, etc.), then the reasons may well be different. The more closely you have to work with them, the more irritated you may feel at their slowness or inability. You still shouldn't go after them for their stupidity, but somehow give them a task they're up to while the rest are working on different aspects.

Nasty, arrogant, abusive, sadistic, offensive, rude, stupid people on the other hand get criticized not for being stupid but as retaliation for their bad acts. In other words, for the same reasons that normal or smart people would be criticized if they did nasty, arrogant, abusive, sadistic, offensive, rude things.

Take care.