I think the OP answers their own question:
On a lot of forums outside of EnWorld (and sometimes, even on EnWorld), 3rd party material is always treated as a last resort and very rarely pulled from.
Emphasis on the word "forums".
That right there is a sample bias, as forums, by their very nature as an older internet medium, are going to favour older and established players.
Forum goers are going to often fall into a couple different camps, with the largest being older fans who remember the 3PP glut of the early 2000s and feel burned by that material. And another large camp being experienced DMs who don't care for 3PP since they can homebrew it themselves.
Looking elsewhere and people are more open to 3PP.
The most obvious place is the Dungeon Master's Guild, that is releasing a contrinual stream of homebrew. There are a few burgeoning 3PP building a rep there. (I'm currently working on a compilation of cool stuff from there; something that I can merge into a single PDF, Photoshop a cover for, and send to Lightningsource for a PoD copy.)
Green Ronin is doing gangbusters with their 3PP campaign setting. Kobold Press is doing very well with a couple must-have books. Goodman Games and Frog God Games are holding their own in terms of Kickstarter Backers.
There is a continual stream of homebrew on Reddit (
https://www.reddit.com/r/UnearthedArcana/) and checking fan sites and even surprising places like
Pinterest reveals a heck of a lot of homebrew.
D&D and RPGs have always had a strong homebrewing community. People are always quick to make the game their own and customize the rules or add personal content. This got a little harder with 3e and 4e—as the game was much more crunchy with tighter balance—but 5e is super easy to design for. I think one of the reasons 3PP are less common is simply that people don't feel the same pressure to pay someone to do it for them.
That said… I think there's another reason: fewer interested publishers.
A lot of small micro-publishers sprung up for 3e, but time showed it was a lot of work for not a lot of payoff. It's hard to get your name out, and your profits are really tied to the success and failure of someone else's product. At any time the publisher can pull the rug out from under you and kill your market.
Just ask Kobold Press who released a big Pathfinder expansion for the campaign setting a few weeks after Paizo announced Pathfinder 2nd Edition. Good like selling the full print run… :/
Plus, the (relative) failure of 4e, the long playtest of 5e, and rise of Kickstarter have really opened up the floodgates of alternative game systems. There was an opening for non-D&D games and people leapt for that openning like never before. I think a lot of people who would have been 3rd Party designers and publishers have opted to just make their own game system and ruleset.
