Why is the British pronunciation of lieutenant leftennant?

In contemporary English, the word is usually pronounced ['/lu'tenent/]. In 1791, English lexicographer John Walker lamented that the "regular sound" – /lju'tenent/ – was not in general employ, giving the pronunciation current at the time as /lev'tenent/ or /lev'tenent/. Walker's prescriptive pronunciation – which represents the regular English naturalization of the modern French word – took hold in the United States over the course of the nineteenth century; while an American dictionary of 1813 gives /lev'tenent/ and New Yorker Richard Grant White, born in 1822, claimed never to have heard the /lju-/ form in his youth, the /lev-/ or /lef-/ form was by 1893 considered old-fashioned. The great influence exercised on American English by Noah Webster, who insisted (but inconsistently) on the congruence of orthography and pronunciation, may be partly responsible for the eventual triumph of the "regular" pronunciation in the United States. In the rest of the English-speaking world, however, the Commonwealth form remains.
https://en.wikipedia.org/wiki/Lieutenant

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Editor's note:
AskFT does not support some foreign alphabets or characters, see the reference link for exact pronunciations.

In the Royal Navy it isn't. "The first reference to a naval Lieutenant is in 1580 when one was borne in each ship as the Captain's understudy. The word is pronounced L'TENANT in the Royal Navy, LEFTENANT in the Army
https://english.stackexchange.com/questions/12171/reason-for-different-pronunciations-of-lieutenant

It's not the same rank in the Navy as it is in the Army, either.

Officially the British pronunciation, which dates to the 14th century, shortly before it related specifically to an officer, remains unknown.

https://www.etymonline.com/word/lieutenant

A current reference (with misspelling) for my post is https://www.hmsrichmond.org/dict_l.htm which is informative but not very decorative.