Why doesn't US English drop the "o" from "subpoena"?

US English usually drops the o from the diphthong oe (or œ).

So for example the o in the UK English manoeuvre, diarrhoea, oesophagus are all usually dropped in US English.

Why then isn’t the o in subpoena also dropped in US English?

Perhaps this is just purely a historical accident. But is there perhaps any "logical" reason for this?

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Google Ngrams suggests that there was a brief moment in the 1950s when subpena may have overtaken subpoena, but since then subpena has died out: