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IgG can transfer from a pregnant individual's circulatory system to the fetal circulatory system (transplacental transfer)

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IgG can transfer from a pregnant individual's circulatory system to the fetal circulatory system (transplacental transfer). This begins in the first trimester of pregnancy and continues to increase as the pregnancy progresses. Most antibodies are transferred during the third trimester.  

Before a vaccine was available Whooping cough (pertussis) was a leading cause of child death. Risk of hospitalization from Pertussis decreases as an infant ages, with the highest incidence of hospitalization occurring between 0 - 1 months of age. This is younger than the age at which the first does of diphtheria/tetanus/acellular pertussis (DTaP) vaccine is generally recommended (2 months of age). Note: hospitalization rates are important because if your hospitalized then you're more likely suffering severe disease symptoms and therefore your chance of dying is also likely higher than those who are infected but not hospitalized.  

To help protect infants from Pertussis before they can receive a DTaP vaccine themselves,  many health agencies, including Health Canada now advise DTaP vaccination in every pregnancy, ideally between 27 weeks and 36 weeks of gestation (though there is no harm to administering it at other times). Vaccination during pregnancy helps lower the risk that an infant will be hospitalized due to Pertussis by ~80-90%.  

If vaccination during pregnancy, already lowers the risk associated with infant exposure to Pertussis (e.g., risk of hospitalization), why do infants need to be vaccinated themselves? Why isn't the vaccination of the pregnant individual enough? (NOTE: you don't need to know anything about Pertussis, except the information give here, to answer this question)