Not enough vaccinate. Although we do a great job vaccinating here and in other industrialized countries, there are many parts of the world where vaccines are not administered at all. In fact, I spent some time in hospitals in the dominican republic & st lucia & saw things there that I would never see here- measles, tetanus & more. Another problem is that sometimes the vaccine does not always imply immunity.
Not available. Access to vaccination requires that the vaccine be available, the population willing, & the resources be present for distribution. In nations in turmoil, with distressed economies this is often never done. Until there is universal access, acceptance & use, we will have pockets of preventable disease. This underscores the need in the us. Air flights can bring any disease to the us overnight.