[jkaptur]

I think this is really interesting - I've heard "guys" used to address a group of mixed genders ("how are you guys?") but it sounds much more gendered when used to refer to a group ("how many guys are in there?" or even "are you looking for those guys?").

[JamesianP]

Perhaps because addressing people comes with more implications and possible angles for communication with those people. So "guys" fits the bill for one desired nuance that became more popular in culture (the "laid back" nuances the article mentions). Previous generations were a lot more formal.