Q: With XML Schema, why do some facets work with some simple types and some don't?
A: The logic of XML Schema facets can only work on simple types that make sense. For example, you could ask XML Schema to constrain a number to be within a minimum or maximum, but you cannot ask it to do so for a word (there isn't a universal sense of min / max for a word).
Here is a list of facets, and the simple types with which they work: