Because people use laptops in all different types of areas and some of those might not be well lit nor do you sometimes have any control of the lighting where you are. External keyboards are typically used at a desk or work space where there is usually adequate lighting.
I don't see Apple doing this. It's not like people aren't going to buy Macs simply because of this. MacBooks are different. People indeed might not buy MacBooks if they didn't have backlit keyboards. My suggestion to the OP... buy a lamp (seriously, I'm not being a smart ass).