First, because they think Joseph didn't translate the Book of Mormon correctly. He was supposed to translate a "Mayan codex" but goofed, so they fix the translation with substitute terms such as this, along with volcanoes, massive stone Mayan temples, tapirs, etc.
Why this particular substitute term?
Because they think that if the "headwaters" of Sidon are south of Zarahemla, the Sidon river must flow north; i.e., it originates in the south and flows north past Zarahemla.
Of course, the text doesn't say that, but promoters of the theory insist on it because their substitute term "headwaters," they think, excludes North America as the setting for the Book of Mormon.
And by excluding North America, they can justify their rejection of what Joseph and Oliver taught about the Hill Cumorah in New York.
This is the type of cascading false assumptions you need to support the Mesoamerican/two-Cumorahs theory.
It's kind of fun to think that a major river such as the Sidon has a specific, identifiable headwaters area.
In the real world, rivers have many headwaters. Here's a nice description quoted by Roger Terry on his blog, here: http://mormonomics.blogspot.com/2017/10/recycled-high-council-stuff.html:
Harline begins his book by observing that trying to find the origins of Sunday is like trying to find the source of a great river. “The delta at the end and the long channel flowing into the delta are easily recognizable. Yet the farther one moves upstream toward the source of the river, the trickier the going: tributaries multiply, lead astray, or go underground. And when finally located, the humble source may bear so little resemblance to the massive amounts of water downstream that one will surely wonder what the beginning can possibly have to do with the end.”1
1. Craig Harline, Sunday: A History of the First Day from Babylonia to the Super Bowl (New York: Doubleday, 2007), 1.
I mention this in case there is anyone who still believes the Mesoamerican/two-Cumorahs theory because of the imaginary "headwaters" of Sidon that require the river flow north.
It's also interesting that Oliver Cowdery used the term "head waters" to refer to a confluence. In Letter VIII he wrote "This gentleman, whose name is Stowel, resided in the town of Bainbridge, on or near the head waters of the Susquehannah river." The origin of the Susquehannah is more than 50 miles upriver from Bainbridge, but several streams flow into the river at Bainbridge.