Bacteria and basically all "bugs" are more superior than we are in evolving into new forms. Who's not to say that there isn't a rare bacteria that can break down xylitol and digest it's non -OH portions? read more
And the mouth is actually a pretty prolific and dirty place in terms of germs. read more
One theory suggests that xylitol-resistant bacteria have impaired adhesive properties. The ability of bacteria to cause decay depends, in part, on their ability to adhere to a tooth's surface. read more