Question

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

Answer 1

to avoid the confusion of different times at different places

Answer 2

According to the GMT or Greenwich Mean Time system, there will always a time difference along with the latitudinal difference.

That's why countries with large latitudinal extension,have a variety local times in it's own land territory.

In order to avoid the confusion of the time difference in a same country,these countries use an average standard time.