An interesting question. We are never told why in the story, but here are some ideas. Historically 'scouts' went ahead of the main party of people for whom they scouted. That sounds vague - armies had scouts to find out what lay ahead; large groups of travellers in wagons might have had scouts to go ahead and check the terrain. The dictionary says a scout was sent out to bring information.
I think this name suits her and is deliberately chosen by Harper Lee. It is as if Scout - especially as she is a child who does not understabd the adult world - is the reader's 'scout' who observes what she sees and relays the information back to us. The American south in the 1930s is alien territory for most of us now, so to have a scout to tell us what is going on is Scout's role in the novel. Through her we discover everyhting we need to know about Maycomb and its prejudices.
Hope this is helpful!