Come on people... There is only one answer, and that is ... MONEY.
Since this is the first version of a new design, Apple wants to maximize their revenues with the new MBA, and still keep the price reasonable. One way to do that is to exclude certain "extras" like LED lights and back-lighted keyboard. Also by putting the back-lighted keyboard back in when they release the next version of the MBA, more people will want to get rid off their old MBAs and get the new model. It's all about the buttom line.