I'm assuming that, as with just about everything else in media, money is at the root of the shunning of "oldies." Stations want to skew as young as they can because advertisers hate anything that sounds "old." And "oldies" certainly fits that description. The question remains, though: Would a station that had been "Oldies xxx.x" and had been playing mostly '60s and '70s with a smattering of '80s really generate substantially more advertising revenue by simply rebranding itself "Classic Hits xxx.x" and keeping the same playlist, or would the rebranding have to be accompanied by the discarding of most of the '60s titles and the addition of dozens of '80s titles?
Oh, and I'm in my mid-50s and work in an office with several people in their 20s. When I talk about songs from the '60s and '70s, those "kids" will often call them oldies. The big songs from the '80s are another matter, in that they never really disappeared from stations that played current songs the way those older songs did. They might not have been played on CHRs, but the teens of the '90s and '00s were still hearing them on adult contemporary stations and in fast-food restaurants' background music -- right along with current chart-toppers. The "kids" use another "old" term to describe those songs: old school.