What? How did you get to that conclusion? That’s not what the article says at all? It says Phyllis Blanchard used the (then incorrect) spelling with an O (while also changing the definition of the term to something most people I think would disagree with) in a paper she wrote and nobody knows why. And it spread from there.
I think you’re interpreting “Today, ExtrOvert is the most common spelling of the term in the United States.” to mean it’s spelled with an A elsewhere, but the author even brings up the Oxford Dictionary (UK) that says that the original spelling with an A is rare in general use. I live outside the US and I pretty much exclusively see the O-spelling.
EDIT: Changed from “incorrect” to “then incorrect” to clarify. She wrote her article before extrOvert entered the dictionary, and - according to the author of the article linked earlier in this thread - her article might have been a big contributing factor for it entering the dictionary that was published soon after.