Most of the optional rules in the 2014 DMG were ideas that got floated in the D&D Next Playtest at one point or another but presumably never met the approval threshold to make it into the core rules. In another world where 5e had followed through on the D&D Next’s initial premise of modular design allowing for a build-your-own-edition, I imagine we might have gotten more products with these sorts of cool “official optional” hacks and add-ons.
@mearls has talked briefly on here about why that idea never really came together. IIRC playtest feedback just showed very little interest in non-core rules. People seemed to like the idea of the build-you-own-edition when they were imagining everything they didn’t like being cordoned off as “official optional,” but nobody actually wanted the rules they liked to not be part of the core rules.