There were a number of contributing factors.
The English, of necessity, had become
“Normanized”, but, over time, the Normans also became “Anglicized”, particularly after 1204
when King John’s ineptness lost the French part of Normandy to the King of France and the
Norman nobles were forced to look more to their English properties. Increasingly out of
touch with their properties in France and with the French court and culture in general, they soon began to look on themselves as English. Norman French began gradually to
degenerate and atrophy. While some in England spoke French and some spoke Latin (and
a few spoke both), everyone, from the highest to the lowest, spoke English, and it gradually
became the lingua franca of the nation once again.