Why doesn't he? His power doesn't come from within, it comes from a deity, the deity can cut off his "supply" of divine power whenever he's being un-paladiny, regardless of what the paladin believes.
1. Because they're a major change, not something that fits quickly and neatly within the classical game.
2. Because a lot of spells and classes would have to be reflavored and maybe even altered mechanically.
3. Even if I did use your color wheel or the allegiance system, I would probably still add something to the effect of "high mental stats add versatility to alignment".