A big part I think is also that there is maybe 5% the amount of the party material out there compared to the high water Mark of 3.x so players really only have Wizards as their material source.
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That's true, compared to the glut that occurred during 3.x, but there are quite a lot of 3rd party products out there nonetheless. A lot (most?) were made using Kickstarter.
Just off the top of my head: Pugmire (Monarchies of Mau), Fifth Edition Foes (Lost Spells, etc.), Tome of Beasts, Midgard, Ebonclad, Mists of Akuma, Ultramodern 5e, Primeval Thule, Book of the Righteous 5e, Arcanis, Dungeons on Demand, Hunter's Mark, Lost Citadel, Lost Dungeons, Scarred Lands Player's Guide, and Under the Mountain. Those are just the ones I can think of, I know there are more.
It certainly isn't the same as the days of 3.x, but there is quite a lot of 3rd party material out there, even outside the DM's Guild.
Edit: I should note that not all of the supplements I listed have been released. Some are still in development.