The same site points out that the number of automatics is increasing outside the USA, and that new designs are changing things.
But really you could just as easily ask “why is the USA so out of step with everywhere else?”
My view here is that I have no expertise in engineering or great interest in the mechanics of cars. But is there a historical element here, too. Compared to the European, US cars in the 50s to 70s tended to be much larger with relatively big engines and designs more suited to longer journeys on bigger freeways. Comfort and stability were more important than agility and performance. Those bigger engines allowed for automatics since the proportional loss of power those gearboxes engendered was relatively small. European cars by contrast had smaller engines and designs more suited to narrower roads with more and tighter bends. Putting automatics in those smaller, more agile cars caused a disproportionate loss of power. Thus European automatics were generally fitted only to larger, top-of-the-range cars where performance and agility were less important than luxury and where price was less of an issue.