Raven Daegmorgan explains why today's fantasy elves are so underwhelming: they're a photocopy of a photocopy!https://t.co/oGrdMXMdMM— Castalia House (@CastaliaHouse) April 30, 2016
What it comes down to is that those following the breakthrough works (and creators) copy what they see and enjoy without comprehending what the hell they're copying, or why the original works as it does. It's the act of kids playing at being their favorite character or archetype without grokking why they dig it, so it comes off as shallow and lifeless.
This also applies to a lot of science fiction and horror as well as fantasy, as we see in so much of the available commercial fiction (be it books, films, series, or games). The remedy is simple: Git Gud by actually reading for comprehension those breakthroughs, then seeking out their sources, and put in the fucking work required to grok it. This cargo cult bullshit doesn't cut it anymore.
The Castalia post's author, Jeffro, has as the foundation of the Appendix N tag a review of that very Appendix from the Dungeon Master's Guide for Advanced Dungeons & Dragons' 1st edition. Start there and get up to speed, then branch out to where your sources are. Know, at a deep level, what you're dealing with or you'll come off as lacking to your readers.