It isn't so much as "best" as it is traditional, and in line with when hens usually go broody and hatch their own. In areas with cold winters, raising them the first part of the year lets them get up to a good size before cold weather, too, to reduce stress if nothing else. It also gives some egg production before they slow down or shut down for the winter. When I was a kid, we bought chicks in February and had them all in the freezer by November or so. I didn't know they could be kept over winter.