Greek philosophers were public intellectuals and, for me at least, reading their texts was somewhat off-putting because of the level of... grandstanding for lack of a better word. ZAMM seems so much more honest. It's framed as a man attempting to pass on actionable wisdom to his son that will make him a better person.
The questions are the same questions people have been asking for thousands of years: what is good? (and derivitives thereof) but Pirsig frames his answers with a very appealing humility.
Having lived off of my writing for a spell, I struggled deeply with the problem of metaphysical quality and no amount of classical philosophy ever brought me the admittedly limited clarity I felt like a got out of ZAMM. YMMV, though.