I think the narrowing of styles in modern fantasy mapping is more to do with the emergence of a basic language, where the symbols or icons we draw or paste on our maps are its glyphs - the basic words of our language. The map tells the layout of the story in these simple glyphs we have subconsciously agreed upon between us... by way of selecting the most aesthetically pleasing versions all the time.
My handwriting is different to yours, but we can still read each other's script (I would hope) because we only ever use the same alphabet, language and word forms - the same glyphs. Its the same with standard/conventional/traditional fantasy mapping. We all use the same glyphs (even though not one of us draws them exactly the same way) to communicate the layout of the story to the widest possible audience (that's why they have to consist of some kind of common language, rather than being utterly unique every time, or no one would understand them at all).
I haven't tried my hand at what most people would call traditional fantasy cartography yet, but if you analyse what I have done instead , you will see almost straight away that without that convenient common language of recognisable glyphs my maps have to be almost spot on photorealistic - just so that people can understand the message they convey.
One thing that strikes me is that people often forget that at any one time in history there will be one massively popular style (or maybe two if one is on its way out and being slowly replaced by a new idea), and that it is actually normal to have this narrowing effect, simply so that we all understand what the map is trying to tell us - which is to communicate the layout of the story.
Variations away from the basic language of glyphs are driven by inspiration and creative artistic experimentation, which act like a force of evolution over time, such that in 50 years time fantasy mapping won't look anything like it does today - just as the vast majority of maps today don't actually look very much like Tolkien's map if you really look at them in a serious comparison.