Why does the action of salivary amylase stop in the stomach?

Hint: he required pH for the salivary enzymes like amylase is 6.7 pH and the stomach has a pH of 1 to 3 which is required for digestion and activation of enzymes. A change in pH alters the structure of enzymes and results in permanent deactivation of enzymes.

Complete answer: The pH at which the activity of amylase is maximum is 6.7.
The change in pH alters the structure of the enzyme.
The enzyme activity depends on the tertiary structure of the protein that gives stability where it has a binding site that precisely fits a particular substrate only.
The binding sites of certain proteins are entirely lost after a shift in pH.
Amylase hydrolyses the starch into simple sugars and thus can make certain foods taste sweet (rice, potatoes, etc.).

Additional information: Salivary amylase begins breaking down starch in the mouth and continues to do so after the food is passed into the stomach and small intestine.
Salivary amylase operates best at a neutral pH but can survive the stomach acid.

Note: The stomach initially has no high acidic pH but takes around 45 minutes to digest using gastric enzymes. As the pH is dropping from 6 to 3, there is effective digestion as every enzyme has different optimum temperature and pH to work.