Because here the violence is much gorier with more humanoid shapes. You see blood slowly spilling out, and the reactions of the Simpsons family. Those reactions being shock, horror and hopelessness. It's tragedy played straight with a slow, deep toned piano track accompanying it, until the ending with the sudden comedic moment of Wiggum having a done goofed moment.
South Park treats Kenny's death as a running gag. It's followed by a punchline, and you know he'll come back anyway. That and the character models of South Park aren't as realistically animated. At least not in the earlier seasons where Kenny dying was common.
TLDR: South Park make sit clear Kenny's death is a joke. The only death played for funnies here was Lisa's.