Question

Why does country having large latitudinal extent have only one standard time

Answer 1

becouse in a country whether large or small it is difficult to rule with two times

Answer 2

We calculate the local time of any place by comparing the longitudinal distance between that place and the Greenwich (0°).

Countries which have a large latitudinal extent,also have a large range of local times according to the GMT theory.

This variation of the time in a single country can occur several disturbances in official and another jobs which are depending on proper time accuracy.

That's why these countries follow a average standard time of all time ranges to avoid such time related confusions.