Why do influenza B viruses usually cause smaller and more localized outbreaks than influenza A viruses?

Influenza A and B viruses differ significantly in their potential to cause widespread disease. While influenza A viruses possess a vast animal reservoir—primarily wild aquatic birds—that facilitates antigenic shift, influenza B viruses do not share this capacity for dramatic, sudden evolutionary change through genetic reassortment with animal hosts.123

Because influenza A viruses can acquire entirely new genome segments from animal strains, they frequently generate novel subtypes to which the human population has little or no pre-existing immunity, leading to global pandemics. In contrast, influenza B viruses are limited to two major lineages, B/Yamagata and B/Victoria, and primarily cause smaller, localized outbreaks. Their inability to undergo the same scale of antigenic shift as influenza A prevents them from triggering the widespread, rapid-onset pandemics associated with their type A counterparts.2341