Analyzing by two parameters, polyphony and range, music written for guitar should use two clefs, G and F. Usually, solo guitar has a polyphonic expression. Writing two melodies in a single stave could seem ambiguous and/or hard to read. More, the basic range of guitar (until fifth fret) adapts easily to the clef F. For analogy, the brass instrument euphonium has a similar range of a spanish guitar and it's really awful, in usual terms, to write music for it in G clef (disregarding taking advantage of the musical education of the person or group).
So, why are guitar pieces written only in G clef? Ok... certainly there are exceptions (I've heard about the existence of composers that have written music for spanish guitar in two clefs, but I've never seen such pieces). Anyway and again, what are the cultural, historical reasons for writing music for guitar using only G clef?