Because a good MTB is a terrible bike and not comfortable for riding around town IMO. So which one does she want? If riding around town, I like Raleighs, but there are others. I'd get one with an upright sitting position, so you can bike with a straight back, and a rack with saddlebags. You could get one with hybrid tires that would let you bike on a dirt path.

But of course, that would be a terrible bike for actual uphill/downhill mountain biking.