It’s common niggle but, as far as I know, nobody is sure of the precise rationale for placing every key, only the broad explanations of the layout that Dvorak published and promoted. The layout wasn’t based only on letter frequency but they attempted to account also for bigram frequency, frequency of repetition within words, frequency with which words are used, and with an objective of rhythmic alternation between hands.

Consider also that it was developed in the 20s and 30s. Nowadays you could throw some moderately hefty compute at almost everything of note written in the English language and come back to an error-free analysis after lunch, but who knows how representative was the corpus they analysed, painstakingly and manually. It might have made perfect sense with their data set.

Ultimately, the English language didn’t evolve to be easy to type, there will always be compromises somewhere, and the English of today isn’t the English of a century ago anyway. I imagine you’d get quite a different layout if you based it on Gen Z text messages or something.

Personally, I can’t help but note that Dvorak’s first name was August.