why do you think the author spends so much of the narration talking about what a kind person lennie is?

Much of the story surrounds the character of Lennie. Lennie adds context to the major conflicts in the story. Both Lennie and George have a chemistry that surrounds Lennie's mental/emotional shortcomings. There is an "otherness" about Lennie which demands more interpretation than perhaps some of the other characters. To really understand the underlying themes of the story, we must understand Lennie.