Because it's expensive. However, they're looking at producing it cheaply from corn cobs, so I hear. (Daily Mail) And it is now on sale in powder form, so who knows. 

This is a lovely little article; I love the way the links are done at the bottom. My only point of contention is "However, this same effect also interferes with the yeast micro-organism and others, so xylitol is inappropriate in making bread, for instance." Sugar added to bread? Is this an American thing, or have I missed something?