"Ambiance" is simply the French spelling of "ambience," borrowed back into English in 1923. English already had "ambience," a perfectly good word for a surrounding atmosphere, but in the early twentieth century writers—especially art critics—decided the French version looked more sophisticated and started using it to describe the overall mood or feel created by an artistic arrangement. So the "why" behind the spelling isn't some ancient sound change or scribal error; it's a case of English reaching across the Channel a second time to borrow essentially the same word dressed in French clothes, because the French form carried a certain artsy cachet.
The broader arc, then, is less about deep history and more about fashion in language: the root idea (something surrounding or encircling) had already made its way into English through "ambience," but "ambiance" got a fresh, separate entry into the language in 1923 specifically because people wanted a stylish, faintly foreign-sounding word to talk about the intangible mood of a room, a painting, or a scene—something that still carries a whiff of chic Frenchness even today.
ambiance (n.) 1923, a reborrowing of the French form of ambience (q.v.), used in art writing as a term meaning "atmospheric effect of an arrangement." also from 1923