I could be mistaken, but I believe that Congress recently passed legislation concerning DST which said, in part, that states must either be all DST or all not. Parts of states can determine what time zone they are in but cannot opt for either going on DST or not. I believe western Indiana has chosen to be in the Central Time Zone and eastern Indiana in the Eastern Time Zone.
If I recall my history correctly, in the 19th century, there was no standardization of time until railroads criscrossed the country. Each area just decided for themselves what would determine their time, ie...12:00 noon would be when the sun was at its zenith over the bank or some such reasoning. Railroads needed to run on timetables, and timetables needed to be standardized by determining the time of day in the same way everywhere. The movement of the sun from east to west necessitated different time zones.
Sorry, sometimes I get carried away. The story of how DST came about is equally interesting (or not), but all this can probably be found by Googling.
Jude