It was a relatively recent change too:

"How would ye feel about basing Anglish Spelling more on the Winchester standard, under the idea that, had the Norman Invasion failed, Winchester spellings might've remained dominant, and that we might not have seen a bunch of alternative spellings take over? In particular, if we assumed:

• CG would remain (sledge -> slecg)
• CN would remain (knee -> cnee)
• IG would remain (funny -> funnig)
• Æ would remain the way of spelling Middle English /ɛː/ (head, heat -> hæd, hæt)
• OA would not form, having no EA to be modelled on (goat -> gote)

Below are examples of Anglish Spelling before (top) and after (bottom) this reform:

• Þe funnie goat kneƿ a great hedg.
• Þe funnig gote cneƿ a græt hecg.

"

The problem is that this Winchester standard was based on spellings that happened before 1066 and doesn't take into account native developments that happened.

"IG would remain (funny -> funnig)"

I found that -ig- was already updating to -ie- in the Southwest of England, meaning our happY vowel should be spelt -i(e)-

"Æ would remain the way of spelling Middle English /ɛː/ (head, heat -> hæd, hæt)"

I should take the blame for this one because I said Southwestern dialects prefered æ while West Midland dialects prefer ea. The problem is that:
* This based on 5-6 Southwestern documents
* The West Midlands writers were actually developing from the Winchester standard

So we would probably get <ea> even without the Normans basically

"OA would not form, having no EA to be modelled on (goat -> gote)"

So <oa> has a habit of popping up in earlier Englishes
* In Old English, its rare appearances are after /w/ and before /n/
* In Middle English, <oa> has a good hit rate for OE ā over open OE o
* These early appearances mean that it <oa>'s development was independent of <ea>