Many reasons but one is quite simple. Self-centeredness and lack of understanding with a historical perspective.
Unless you are over 60 you simply don't remember the fear that these diseases instilled in the public. You can dismiss it, saying "I don't know of anybody affected in the least by polio".
Sure, I'm over 60 (by a little!) and I never knew anyone with polio, but my parents, grandparents, uncles, aunts, in-laws, etc. all shared stories of classmates and family members who got sick and spent their life in an iron lung or died. Or lost their hearing from a virus. Or lost part of a lung from pneumonia brought on by a virus.
Take a walk in an old cemetery and see all the tombstones for children who died from outbreaks