As far as I can tell from accounts of how dishes came into being, it was a combination of France becoming a major cultural power that exported fashion and architecture as well as food and France's development of court cuisine as a dedicated specialty with an almost academic structure of training and discourse. Instead of having the nobility and rich eat more expensively prepared forms of what most would eat (albeit with different proportions and frequencies, with the British nobility eating roast beef almost nightly and the peasantry eating it on holidays), the royal tables of France had completely different dishes made with different techniques than other contexts. Thus, fine dining (rather than holiday roasts) emerged in the west.