It might not be an active preference thing, but because '79 was the last year to make a Standard with the 36 spoke wheels. The later were either 64 spokes on the Heritage and are considered by many (myself included) to be ugly, or mags. I don't really care much about the XS timeline, but that seems fairlyaccurate to the best of my knowledge.
Mostly though, it's probably just because the later the production date, the more of them remain. I'm not a XS Historian, but I'd also be willing to bet that '79 was one of it's highest production years.
EDIT: I'd focus on anything from 77' on, if you have a choice, just because all of the bugs were worked out of them by '77. Although I really don't think you can go wrong with any of them. They were great from the beginning and only got better.