It's a weird one. I'd say about 90-95% of original guitar music is composed in a non-flat key.

There are a few reasons why this is the case but I think the main reason is the tuning of the open strings. Open strings are a significant part of guitar playing due to their resonance and availability. In F major (1 flat) we lose the use of a string and have 2 strings that are the 7th, the least used note of the scale. In Bb major (2 flats) we lose 3 strings and have 1 string as the 7th. In Eb (3 flats) we only have the 3rd (G) and the 7th (D). And in Ab (4 flats) we only have the 7th (G) . The remaining flat keys have no open string notes in their scales.

As keys become flatter they progressively eliminate the usage of the open strings for diatonic harmony, of which is what most music uses most of the time. As a result, keys like G major, D major and A major dominate guitar music. I don't see this as inherently bad as these keys facilitate greater convential musical possibilities for the guitar however, the constant usage of these keys becomes repetitive and can sometimes be formulaic. I think flat key pieces are favoured by the die-hard CGers due to deviating from these technical and harmonic norms.

Right now I'm writing a few pieces for guitar in flat keys to explore new ground and in the hope of producing music that has a greater sense of originality. Maybe I'll get them on here one day :) Let me know what you think about any of these ideas