Why? Good Question...
Everybody who has ever written about Gallatin County notes that it was named for Albert Gallatin, a Swiss emigrant who made good in America, and became America's longest-serving Secretary of the Treasury, under two terms with Thomas Jefferson and two terms under James Madison. Gallatin had additional accomplishments, and he's an excellent pick to have named the county after.
But . . .
Literally none of the things for which Gallatin is known today were things he accomplished before the county was named for him. They were all done after the county was named. The county was named after him in 1798. He didn't begin his tenure as Secretary of the Treasury until 1800.
So why'd they name the county after him?
The short answer: nobody knows, but we have two guesses.
1. Gallatin was involved in settling the Whiskey Rebellion [Wikipedia] in Pennsylvania. The tax on whiskey would have been as unpopular in Kentucky as it was in Pennsylvania, but Gallatin's role was relatively minor.
2. Father Albert Ruschman notes that Gallatin was a Swiss emigrant. He also notes that Switzerland County, Indiana obviously has a strong Swiss connection. Were early settlers of Switzerland County, Indiana (Indian Territory when Gallatin County was established) previously residents of Gallatin, and were looking for a Swiss born man of note to name their county after?