Marandahir
Crown-Forester (he/him)
The Elemental Evil guide was not playtested, and SCAG had several elements that were not playtested in advance, as did Wildemount. And it could be that the limited tests for MotM provided enough data to make the case for it. Also, MOTM might itself be the test, to see how it is received as a variant, before the reiterate core rules changes based on feedback.
Elemental Evil and SCAG are both books that WotC has sort-of admitted as mistakes to license out to Green Ronin, built on the idea that they were so busy with 5e rollout (to the point that they delayed the DMG) that they needed more hands on deck. Elemental Evil Player Companion was SUPPOSED to be a hardcover book originally, with the idea of a player book and an adventure module for each AL Season, but there wasn't enough good of substance there, and WotC cancelled the hardcover and turned it into a free to download (and cheap to POD) pdf instead. And after SCAG, they dropped the Green Ronin deal altogether.
And then, notably, while the lineages and spells of Elemental Evil got picked up later on, very few elements from SCAG have been rescued from the scrap pile: Bladesinger took years to be rescued and it was overhauled for Tasha's. Sun Soul, Swashbuckler, Mastermind, and Storm Sorcery made it into Xanathar's, and Duergar and Deep Gnomes have appeared here and there, but Path of the Battlerager, Oath of the Crown, Banneret/Purple Dragon Knight, Way of the Long Death, Ghostwise Halfling, Half-elf variants, and Tiefling variants have all been abandoned. Worse off is The Undying Patron for Warlocks: it's been REPLACED in terms of narrative function by The Undead Patron in Van Richten's Guide. They never would have created two such similarly conceptual Otherworldly Patrons for Warlocks if they didn't intend to leave the (admittedly underwhelming) Undying Patron in the dust.