Long story short, "viruses" include tremendous diversity within that categorization just as all other species do. Most of them are not easily stopped with vaccination. Coronaviruses in particular are legendary for their rapid adaptability. We are now watching this in real time, but it has been known for decades. The current strategy was a non-starter before it was ever launched. Doesn't stop them from trying, however, creating misery for billions with their attempt.

Small pox in particular had become quite specialized. It didn't have an alternate reservoir (another species) where it could hide. As to polio, look at the results of Bill Gates' work in Southern Africa, where recent polio outbreaks are actually attributed to precisely the vaccine that was supposed to prevent it.

https://fort-russ.com/2020/09/major-who-admits-gates-backed-vaccine-caused-recent-polio-outbreak-in-africa/