Maybe because he was that dense? I mean after all. Jesus had to ask him three times if he loved Him. Then Jesus even told Peter to “get behind me Satan”. Peter even denyed Christ.
That was a stupid answer and not worthy of anything more than a remark as to its stupidity.
In all honesty Jesus did say “are you so slow of learning”....so yes they had difficulty at times understanding His meanings....concerning that time i can see why after being indoctrinated as Jewish...or pagan...for the crowds who heard. But one thing we know...it is much easier for someone who as had no catholic instruction to understand the Christian truths as Christ taught us.... than those who do.