the problem with games like Civlization, is the the original games used a 320X240 resolution, which is significantly larger than the GBAs 240X160. To port games like that over means creating them from the ground up, art assest cannot be reused without being edited, and so on. So it isn't cost effective to port them to GBA.
I am hoping that nintendo sees fit to use a 320X240 screen in thier next handheld, the possibilties would be endless.