Why does tooth decay start when the pH of mouth is lower than 5.5? (AS1)

Tooth decay starts when the pH of mouth is lower than 5.5 because:

Tooth decays which are made of calcium phosphate are the hardest substance in the body.

It does not dissolve in water.

It gets corroded when the pH in the mouth is lower than 5.5.

Acids are produced by the bacteria present in the mouth