They say they would not be able to deliver the best user experiece which in their case means TouchWiz. Many people report that TouchWiz is a resource hog.
Some articles even suggest that contrary to Samsung's statement NOW WITH KitKat TouchWiz would run smoother.
I believe it's money:
-rewriting&optimizing TouchWiz for KitKat $$$
-pushing update on servers $$$
-not selling bigger phones ("want KitKat? Get S4!") $$$ (for them)
It's a good business decision money wise but morale wise ... Not.