For emergency operations, busses offer a flexibility rail will never be able to match.
Evacuating by train would require evacuation to the train and then transportation from the train to shelters. This is not a problem with busses--they can go directly from collection points with mobility-impaired populations and take those people directly to shelters, while trains would need busses at both ends of the journey. The US charter bus fleet is large and mostly continuously in use, so a large fleet already prepared and equipped for journeys to and from disparate locations exists and because it is in regular use, the only costs to governments are during the crisis.
Evacuating by train would require evacuation to the train and then transportation from the train to shelters. This is not a problem with busses--they can go directly from collection points with mobility-impaired populations and take those people directly to shelters, while trains would need busses at both ends of the journey. The US charter bus fleet is large and mostly continuously in use, so a large fleet already prepared and equipped for journeys to and from disparate locations exists and because it is in regular use, the only costs to governments are during the crisis.