I think another big thing is this:
The 5E market is very hostile towards its homebrew and third party content creators.
"But Shardstone, look at all these kickstarters! Look at how many backers they have!"
While there is certainly a renaissance currently underway, that renaissance is a symptom of 5E's absolutely massive amount of players. If we say that modern D&D has 15 million players, then 1% of those players is 150,000 people. That's probably close to how many people currently engage with 3rd party content, if not a little more.
The vast majority of the playerbase doesn't respect this content, however. You see it everywhere. Homebrew etc etc is treated as if it is always inferior, always terrible, and that it has to actually not just be perfect, but sublime in order to justify being used. This is a result of the 3rd Edition OGL and the lack of quality there, and also the fact that 5E is very close-handed about how its game is designed, which means a lot of content creators either have to work around 5E's design paradigms or try to intuit them and risk getting them wrong.
Regardless of the reasons, the 5E Market as a whole doesn't give much attention to those that aren't first party. I can name a litany of products right now that I think are superior to almost every single 5E product, but they not only make a fraction of the sales, the wider consensus is "Because it is third party, it already is low quality."
So not only are publishers forced to try and tap the 5E market, they have to deal with an absolute storm of negative opinion and hostile attitudes while trying to market their current heartbreaker or deeply impassioned project and at the end of the day, they still get a fraction of a fraction of a fraction of the respect, attention, or praise that WotC does.
This post isn't to say that WotC content is always low quality, and that no amazing things have come from WotC. This post is to point out that it truly feels sometimes like a winless position to take, getting into TTRPG publishing, and to provide some insight as to why I think the rest of the market struggles as it does.