y is the south called Dixie?

There are 3 basic stories for the word origin.

1. Dix bank notes. This is the most common story, but it is totally inaccurate and in fact was promoted early in the 20th century by a New Orleans bank.

2. Mason/Dixon line. This version has frequently been presented without much popularity, but it was an argument for previous use of the term for the South. There is no indication that the South was called Dixie prior to the song.

3. Mr. Dixie's Plantation Basically this is a mythic story, as Emmett himself said when asked: that there was a kindly master who freed/sold his slaves and they longed to go back to Dixie's Land. This was the most widely used interpretation in the mid-to-late 19th century, although sometimes with a specific place as "historic" accuracy. This is in fact the correct origin, but it is a fairly complex story (you have to wait on my book for the full details.) The word was used prior to the song as a minstrel show male name, there are indications of "real" (as opposed to coaled) Black men having been named Dixie, and African-American use of the term. The idea of a mythic place has had long lasting impact in how the term continued to be applied.

The word was attached to the South after the song, and it spread quite rapidly so that by very early in the War the South was known as Dixie much to the alarm of the Southern elite.

Back when currency was issued by large banks rather than the federal government, many 10 dollars bills in the south were issued by a Louisiana bank that still had strong French roots. On the back of those 10 dollar bills was the word "dix" (ten in French). Louisiana, and later the South in general, were often referred to as Dixie land because of this.

its short for the "mason/dixon" line thet you have to cross to get there.