At higher latitudes, day length collapses fast in October and November, and fawns dropped too late in spring won't survive their first winter. Selection has tightened breeding into the first week or two of November so fawns hit the ground in late May and have a full growing season to put on weight.
At lower latitudes, the photoperiod change is slower, the winter survival pressure on fawns is much lighter, and breeding can spread over a longer window. Genetics also play a role — Deep South whitetails carry different historical breeding-window genetics than Northern deer, and translocation of Northern deer into the South in the early 1900s scrambled rut timing in some states for decades before settling.
In practice, this means your hunting calendar in Alabama or Mississippi is a different shape than in Iowa or Pennsylvania. December rifle hunts in the South can run during peak rut. October hunts in the North can land squarely in pre-rut.
For the specific peak rut dates by state, county, and DMU, see our state rut date pages — Southern peaks vary substantially even within a single state.