American culture is, I believe, becoming more homogeneous over time as a result of information technology. Unless you're from either rural Appalachia or some deep part of the South, you just don't have an accent. Or rather, you have a generic "American" accent.
Yeah I'm absurdly good at not speaking in my native Appalachian accent, opting instead for a sort of "generic midwest." Unless I'm tawking to ma day-ed-dy, n' then all bets is awf."
My sister and I cover so well and so often that none of her four children who are literally being raised in Charleston, West Virginia has even a trace of our accent.
I came here to say essentially the same thing. It's hillbilly code-switching; if you speak without an accent most of the time, but then meet someone who speaks that way, you can put the accent "back on" again and immediately gain a massive boost to your charisma stat when you need it.
I do it all the time at work. No accent until I'm providing care for someone with a southern accent, then bam! "Well, ah doan know whai inneewon wooden lissen to me!"
YEP. Same. It's always entertaining at work to watch a colleague who's never heard my accent before hear me speak to a patient from Appalachia or the south
Information technology - and contact, in general - is at most the "grease" smoothing out this sort of change, not the cause itself. The actual cause is usually social and ideological. Nationalism, lower prestige associated with the dialect in question, this kind of thing.
That happens because we use speech itself to highlight who we identify ourselves with.
Another thing to consider is that a lot of traits usually associated with Southern American English ended associated with African American Vernacular English instead - non-rhoticity, gerund in /n/ instead of /ŋ/, etc. So at least in this case this might have a component of racism (I do not know if it does), where locals would rather be associated with whites from another region than with black people in their own region.
The article also mentions another component - generations. People not wanting to speak like a boomer, and attributing the traditional dialect to boomers.
For contrast: in Brazil I've been informally noticing the opposite trend, at least in my region, with kids sometimes having a thicker accent than even me (and my own is already thick), with the accent levelling being mostly local between the rural zone and nearby urban centres. Info tech is still everywhere here, and those same bloody kids don't stop staring those bloody phones because of their bloody zapzap WhatsApp, but they still prefer to speak like locals instead of adopting somewhere else's accent. (I'm mentioning kids because dialect change is usually cross-gen.)
It's still weird for an accent to vanish so fast for a country the size of the US. I'm from the country the size of Maine, and while the accents got much closer during the 20th century due to standardized education and mass media, I can usually identify where someone comes from quite easily. (And since in the last 20 years or so there has been more representation of regional accents on TV so I don't think they are going anywhere any time soon).
For a cultural feature to fade fast, there needs to be a conscious effort from people to avoid it.
Why would you use an accent when it actively gets you associated with idiots / racists / conservatives?
There's a lot of people who have been hurt by Southern culture, so naturally they reject that accent. A lot of decent people reject the accent because they don't want people to assume they're one of those Southerners. There's also the aspect that you end up talking to people outside your region a lot more often than in the old days, so you learn to switch it off because people outside your region might not understand you well.