Well.....just throwing out a little IMHO here.....perhaps they didn't have the "apptitude" for learning how to read/write music. I was a voice major in college, and still sing/play guitar in a World Music band. Of course, solfege, basic piano, music theory, etc. were all required courses, but somehow, I never "really" understood the whole reading music thing, and had to take solfege more than once to pass it. I have a very good ear, and can pick up anything I need to learn by just listening to it. I can tell when someone in the band is "off" but I can't tell them whether it is a C or a C sharp they are singing wrong. So, perhaps others are the same?
Hugs, Ashara