Question

why does Magnesium and manganese only produce Hydrogen when reacted with HNO3

Answer 1

Metals do react with nitric acid. However, hydrogen gas is not evolved when a metal reacts with nitric acid. This is because nitric acid is a strong oxidising agent. It oxidises the H2 produced to water and itself gets reduced to any of the nitrogen oxides (N2O, NO, NO2).

Answer 2

because these metals are highly reactive metal as well as they react with Dil. hno3 to produce hydrogen