"We cant imagine why US that has multiple time-zones already in a single country"
Because "The USA" spans from GMT+10 to GMT-10. Your Solar Noon only varies by an hour between Kolkata and Mumbai. Without time zones, the continental U.S. would see a solar noon difference of three and a half hours. And that's not including Alaska, Hawaii, and Guam, which gives us a 20 hour span. You want all those places to be in the same time zone?
Because "The USA" spans from GMT+10 to GMT-10. Your Solar Noon only varies by an hour between Kolkata and Mumbai. Without time zones, the continental U.S. would see a solar noon difference of three and a half hours. And that's not including Alaska, Hawaii, and Guam, which gives us a 20 hour span. You want all those places to be in the same time zone?