The story lines have been shortened, and important details have been left out.
Explanation:
As this is an opinion question I can only give my take on this but as I read reviews I've noticed people mention things that have been left out a lot, so most people are disappointed in movie version of their favorite books because important details are left out.
Since Kenny had to catch Rufus up. Rufus thought that Kenny was his friend. Rufus didn't come to Kenny's house because Kenny laughed at the mean comment Larry Dunn said.