I think it might be more that the industrialization of agriculture made rare flavors far more commonplace.

Even real vanilla isn't all that expensive or hard to come by anymore. Virtually every grocery store has it in quantities that will suffice for anything but full on commercial needs.

So it has become a default flavor, exactly because it is both cheap to acquire and popular to consume. Not unlike chocolate (which, I might argue, is more vanilla than even mediocre quality vanilla).