Humm, honest question, why did the royals from the Norman conquest on start speaking french?

I guess the Normans while not being "ethnically" Frank or Gallic at the time, had already been absorbed into "Frank Culture". They were also loose vassals to the king of France at the time as I recollect.

"Ah, l'Angleterre, cette colonie française qui a mal tourné." - G. Clémenceau

--Edited for clarity