It has to do with when the sound shift occurred. The change to /wa/ in what is now Modern French (which is a langue d’oïl not a langue d’oc like Provençal) happened after the Norman Invasion. From Wikipedia:
In some contexts, /oi/ became /e/, still written oi in Modern French. During the early Old French period this sound was pronounced as the writing suggests, as /oi/ with stress on the front vowel: /ói/. The stress later shifted to the end position, /oí/, before becoming /oé/. This sound developed variously in different varieties of Oïl language – most of the surviving languages maintain a pronunciation as /we/ – but literary French adopted a dialectal phonology /wa/. The doublet of français and François in modern French orthography demonstrates this mix of dialectal features.
Also, even though Norman French was also a langue d’oïl (albeit with other influences), do not assume it underwent the same sound shifts as Parisian French.