Why don’t we have a vaccine for every infectious disease?
For some diseases, it’s a question of difficulty.
HIV, for instance, can insert itself into the genetic material of your cells and hide there undetected. Other viruses, like dengue, have multiple strains, which makes it very difficult to create a vaccine that covers them all. With other pathogens, the problem is mutation: continual morphing means that the immune system essentially encounters a new threat every single time, making past exposure count for nothing. This is why the common cold is so problematic.
https://wellcome.org/news/why-do-we-need-vaccines