I think "y'all" is becoming more common at least in part out of an interest to avoid "you guys," which people perceive to be overly gendered. I notice (in my own speech and others') "folks" replacing "guys" in other contexts, for the same reason. The fact that both make everyone sound a little more casual, and a little more southern, is probably mostly a side effect -- general American English just doesn't have obvious gender-neutral words to fill those semantic functions.
