The reason he might have thought to introduce the characters and scene in that order is because he wants to give the reader the understanding of mindsets of the people in that town and what it would be like if you were to live there for a long time. He wants to introduce the type of everyday experience in a town that is so small in which everyone knows eachother. If you were in the town, somebody would knew you.
He introduced the people next so you could understand stories and how someones personality would be like, the way someone would act in a certain situation if anything where to happen to someone in the town.
He then introduced the Clutters because you already have an understanding of the town. The clutters were well known so what happened to them shocked the rest of the people in the town because they were popular and nothing bad seemed to happen.
Then the narrator introduces the killers last because they want to introduce the murder and who were going to murder them. They also give importance on to why the murder matters in that town.
