A: Because it is made with real cane sugar. (cheer!)  
We use corn syrup. (boo!)
(doesn't that just sound so...American? 
I think they are going to make rockets out of corn pretty soon...)
Well that begs the question: 
 Why don't we make Coke with real cane sugar, too?  
Are Americans just slow on the draw?
Three words: 
No Free Trade.
You see, we don't have the resources to grow sugar cane here.  
(Except for a few farms in Louisiana...)  So we have to trade for it.  BUT our brilliant government puts down restrictions*. Like 
tariffs. (Boo!)
We 
could get sugar for 
14 cents a pound, 
but the tariff increases it to 
24 cents a pound.  That is too expensive for our Coca Cola Corporation.  See why we revert to good ol' corn syrup?  We are suffering here!
Get this: Those 
Louisiana sugar cane farmers? Think they live the life of a modest farmer? Think again.  Try a yearly income of 
 $1 MILLION. I am switching from engineering to farming.
So, uh, what dwindling sugar cane farming industry are we trying to "preserve" here?  
That's what I thought.
You should 
really be booing now.
[ Boo squared! ( = boo!^2   = BOOOO!) ]
Ok, so if we took the tariff off, the overwhelming supply of sugar probably would wipe out the Louisiana farmers.  But that's ok.  Our economy would actually be better off as a whole. And they wouldn't be out of a job, they would just relocate...to....ya know....corn farming....
I hear that the industry is absolutely booming. 
(cheer!)
*Economists are still baffled as to why our government does this.  It doesn't make economical sense.