There is a handful of reasons for this (anti-vaccine movement, shorter duration of immunity with acellular vaccine, etc…). The following link has a several minute clip that discusses the issues.
I hope you will find it worth your time to view and share with others. Don’t forget: in the “pre-vaccine era,” whooping cough was referred to as the “100 day cough” and few people of any age would want that! Remember, VACCINES EQUAL PREVENTION and can be life-saving!
“How whooping cough became an epidemic in 2014” MSNBC News (June 17, 2014)