I remember reading that it was Italian chefs working in French aristocratic homes that created much of what we now think of as French cuisine. No idea how true that is but it seems plausible to me.
Its a mystery and nobody seems to have the answer. Note that the Italian food is also varied and their regional variations are also distinct. Like the French they also have their wines down to a deeper level. But what is the trigger? And the Roman conquered Britain but why no food influences to talk about.