It's because the Law vs. Chaos axis isn't really an ethical spectrum; it's a collection of personality traits. Whereas many cultural and philosophical beliefs can be projected onto the Good vs. Evil axis, Law and Chaos are collections of personality traits*. And because personality traits are not at all mutually exclusive, determining what's Chaotic, Neutral, and Lawful can be nigh-impossible unless a creature falls neatly into one of those three boxes.
For example, I keep my word and largely tell the truth. When I'm on a team, I play for the team and can be relied upon to do at least my share. I can also be slow to react to new situations. However, I also strongly believe in personal freedom and don't particularly like being part of a team in the first place. I have a facile mind and imagination, and understand new concepts quickly. I don't believe in change for the sake of change, but nor do I believe in continuity for the sake of continuity. I have near-endless patience for putting things in order, but almost none for things that I consider trivial or politicking.
Am I Lawful, Neutral, or Chaotic? I think you can see the problem.
*2e Law and Chaos are more philosophical/ethical, but the only vestige of this in 3.x is the "dedicated to balance" option for True Neutrals.