To incentivize vaccine manufacturers to produce vaccines that are affordable to the general populace.
In the US you do still have some recourse for injuries caused by vaccination: "The National Vaccine Injury Compensation Program is a no-fault alternative to the traditional legal system for resolving vaccine injury petitions.
It was created in the 1980s, after lawsuits against vaccine companies and health care providers threatened to cause vaccine shortages and reduce U.S. vaccination rates, which could have caused a resurgence of vaccine preventable diseases."