The reason being that it is easier to keep a definite order to what one is reporting, remember the order of the information that was gathered, who said what, and when the information was found.
When "In Cold Blood" was first published in January 1966, Hickok and Smith had been dead for less than a year. The murder and trial had garnered big headlines, and many readers probably knew the details of the novel before they began reading it. Capote had to make it interesting even to people who knew the outcome; the book had to be good literature as well as be informative and accurate. The novel is saturated with details that would never have been included in a newspaper. Moreover, the details are carefully picked. Knowing that Capote compiled 8,000 pages of research, the book seems to be a very carefully edited selection of facts and descriptions. For example, very little is said about the two older Clutter daughters, although Capote doubtless interviewed them. He left them out for artistic reasons, to keep the reader focused on the story. This shows that the facts of the Clutter case were building blocks for what was ultimately a creative work.
I feel that Truman Capote was an ass. In writing his "non-fiction novel" for the entire world to see, he proved that he had no empathy towards anyone, except Perry Smith, the one who killed all four members of the Clutter family. It is common courtesy that when someone's relatives, or anyone involved in the death of someone dies, that one not talk blatantly about it. In this example of Capote releasing the "novel" right after Richard Hickock and Perry Smith, he did not wait for the families of Hickock and Smith to grieve. He also did not allow for the Clutter friends and family to search for the peace that they were most likely looking for when the murderers were hanged. .
In arranging the facts of the Clutter case into a novel, Capote gave them a number of meanings.