Polio's death rate was around that of the flu. It was so feared because of the paralyzed survivors, but even that was <1% of infections, not the huge percentage as commonly portrayed nowadays. Around 3/4 of those infected never even had symptoms, and most of the remaining 1/4 only had flu-like symptoms.

On top of that, like the other one GP mentioned, one of its main methods of transmission is feces contamination. It's possible hygiene was also a major factor in stopping it.