It's not just the US, BTW, I find it even more pronounced here in Canada. Listening to Canadian radio broadcasts from the 60s ... it's almost like they're from the UK. Different cadence and quite a bit more British Isles going on.
We know accents change over time. Just listen to radio from the 30s or 50s to hear changes between then and the 70s or 80s. Then again to now. This is a human phenomena, and not one that would be isolated to just the US. However, for both Canada and the US immigrants should have an outsized impact on accent shifts compared to populations that are more stable.
I'm curious to know the impetus for the change over time.
Go watch a Woody Allen movie from the 80s and you'll hear the same speech patterns.
I speculate the fact video is used differently today than in 1984 is the culprit. Video today needs to be entertaining and compelling. It must keep ones interest. In 1984 there were fewer (video) resources and so they could take their time, and allow the viewer to experience and interpret the material on their own, without the worry or risk that the viewer would "change the channel."