Vaccines should not be made mandatory because vaccine mandates are inherently unconstitutional.
For instance, the First Amendment text states that Congress can not make laws "respecting an establishment of religion, or prohibiting the free exercise thereof."
There are many people who do not want to be vaccinated because it is against their religious values and morals.
Several religions oppose vaccines and mandatory vaccinations. One such example is The Church of Illumination; they state that “the teachings of the Church unequivocally affirm that injections of vaccines and inoculations are a violation of these biblical teachings… Immunizations and vaccinations are a form of blood pollution."
If the government were to force these people to get vaccinated, the government would be imposing on their individual rights.
Vaccines should not be made mandatory because such mandates will interfere with rights guaranteed by the Constitution.