I think the reasoning for this is less "pretend 4e didn't happen" and more WotC's insistence that Nerath isn't actually a distinct D&D setting but rather just "the" D&D World.
So, from the standpoint of a fan, the Nentir Vale and Nerath and all its associated proper nouns and cosmologies and histories and events are absolutely a distinct, different, unique, D&D setting. It's among the pantheon of Basic Fantasy Settings -- Greyhawk, Forgotten Realms, Dragonlance, and now Nerath.
But 4e took pains to articulate that Nerath = D&D. It wasn't that Nerath had a sucking Abyss at the center of an Elemental Chaos, it was that D&D had a sucking Abyss at the center of an Elemental Chaos. It wasn't that Nerath was a world of points of light in the darkness, it was that D&D was a world of points of light in the darkness. It was part of 4e's eager desire to re-define and re-brand everything that it had One True World and every product written in 4e outside of specific campaign settings was about this One True World which was also definitive of D&D. And even the 4e campaign settings couldn't escape it entirely: Dark Sun has a Feywild and Returned Abeir brought the dragonborn, because they are D&D worlds and D&D worlds in 4e all had Feywild and dragonborn because they were just variations on Nerath.
The reasoning for this re-definition is, I imagine, deep within the re-branding efforts that caused 4e to re-define a lot of things. But the fact that this was how Nerath was billed originally makes it hard to disentangle from "default D&D."
I just think they haven't quite figured out how to present the world in a way that isn't "default D&D" yet.
Like, if you look at the worlds of D&D, they all have kind of unique selling points -- FR is all about the history and magic of the place, the intense and dramatic events. Dragonlance has the defining epic adventure of D&D at its heart. Greyhawk is a bloody Swords & Sorcery world of wars and kingdoms in conflict. Nerath....you could maybe highlight the "points of light" aspect or the cosmology or something, but it's a little harder to disentangle from Any Random D&D World. And their efforts haven't been focused there quite yet. It looks like bits of Nerath are making it into 5e's game as options (Turathi tieflings, dragonborn, etc.), not as a cohesive whole.
I bet that one of the most useful things you can do to get WotC to pay attention to Nerath again is to actively continue fan support of the setting yourself. Get together a community of folks who totally dig Nerath and who want to catalogue its information and develop rules in 5e for things that were unique to it (like elemental demons!). If there looks to be a demand for it, I'm sure it'll go a few pegs up WotC's concern-o-meter.
For me personally, I think I'll like Nerath a lot better as a distinct D&D setting than I did as the One True D&D setting, so I'd be interested to see what big fans of the setting might produce that make it stand out from the DL/GH/FR crowd. The first thing I'd like to hear is "Why would I play in Nerath instead of in any other setting?"