Question

why nitric acid is not used in the preparation of hydrogen​

Answer 1

Answer:

This is because nitric acid is a strong oxidising agent, and oxidises hydrogen gas to convert it into the water.

Answer 2

Nitric Acid is a very strong oxidizing agent. It oxidizes the Dihydrogen Gas produced during the reaction with metals to Water. That is why Nitric Acid cannot be used for the preparation of hydrogen gas.
However, metals like Magnesium and Manganese still produce Hydrogen Gas but with very dilute nitric acid. Only 1% dilute and cold nitric acid reacts with magnesium and manganese to liberate Hydrogen gas.