Why didn't French engulf English entirely?
English was spoken by 90% of the population (poor).
French rulers of Britain became cut off from French identity - nobility married English speakers and noble families learnt English from mothers/nurses - ruling classes became bilingual.
The bubonic plague in 14th century - latin speaking clergy were reduce by half in some areas (communal monasteries) - were replaced by English speakers.
Survivors of plague had greater opportunity and power - English speakers became to occupy education, state, laws, royalty. English used to rebuild idea of nationhood.