Why does "broad" not rhyme with "boat"?

Question

The word "broad" is pronounced /brɔːd/ (some US accents: /brɑːd/) instead of */brəʊd/. The spelling -OA- somehow suggests that these words are closely related and/or were pronounced the same at some point. In simple words, broad rhymes with caught, thought, sought and doesn't rhyme with BOAT, COAT, LOATH, ROAD, FOAM etc.
"Broad" is from Old English brād and its Old English pronunciation was /brɑːd/.
It seems that boat, road, loath, foam and broad had the same vowel sound /ɑː/ in Old English. All of them were spelled with ā and Wikitionary notes that their pronunciations were with /ɑː/ vowel. Why then is broad not pronounced the same as all those other words?
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About a week ago I was reading a book on internet archive (whose name I have forgotten now and I am unable to find it again) which had an entry on "broad" and "abroad". I was lucky enough to take and save a screenshot from the entry. But I do not understand it well.

herisson · Accepted Answer

The pronunciation of broad with /ɔː/ instead of /əʊ/ is exceptional. Most words spelled with oa followed by a letter other than "r" are pronounced with /əʊ/ (/oʊ/ in American English), which developed regularly from Middle English /ɔː/. A previous question asking whether there are any similar examples of oa being pronounced /ɔː/ turned up no similar words: Is the pronunciation of "oa" in "broad" unique?
There is not a clear reason for broad not being pronounced /brəʊd/. As the extract you quote mentions, it is possible that the use of /ɔː/ is related to the r in this word, but that's not a perfect explanation because there are other words with r such as road, broke, groan that have /əʊ/ and not /ɔː/ in modern English.
Great, break, steak are similar exceptions to the usual development of /iː/ in words with the Middle English vowel [ɛː] and the modern English spelling pattern -ea-.

Why does "broad" not rhyme with "boat"?

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