This selection is a economics 101. There are no symbol generators with the IS&S displays and thus the replacement cost associted with the SB is eliminated. SB's are very expensive and believe it or not, they are getting hard to find as the OEM systems become more dated. Might not be the perfect answer, but still not a bad decision on DAL part.
Boeing offers a system to upgrade the OEM 757/767 flight displays. Have no idea the cost but I bet it's at least five times more, if not one just one dime?