This is what I've heard it comes down to. A lot of 3PP are no longer putting their stuff up on Paizo due to the cut that Paizo takes. DriveThruRPG and Open Gaming Store seem to offer much better terms.
DriveThruRPG takes 35% of the sale (or 30% if you sign an exclusive contract)
Paizo takes 25%
The Open Gaming Store takes 20%.
Compare that to having your own webstore that takes 2.9% + $0.30/transaction. This is why Kobold Press releases much more in their own website than at DTRPG.
Paizo's site is a pain in the butt, especially the publisher side of things. Not only that it offers the least details about sales, details that can help make sales. That would be forgivable if the sales are there, which they are not anymore and haven't been since they redesigned their website. Frankly I suspect that the biggest thing that hurt sales at Paizo's site is the elimination of the store blog. Before the website design, they had a daily blog advertising some product in their store and links to the recent blogs were emailed to their entire customer base every week. They had a regular 3PP Roundup blog that made a difference in getting sales there. They stopped doing that not long before the website redesign. When that ended, sales dropped.
DriveThruRPG has tools in their site to get noticed: banner ads, featured products, an email customers tool, etc. The Open Gaming Store has d20pfsrd, the 5esrd, the 13thAgeSRD, and more that let a publisher post their material on their site, in a "try before you buy" idea. The only thing Paizo offers is their forums. With customers shifting towards Facebook and Discord, a forum alone is not as useful as it use to be. Their tools for encouraging sales at their site are not keeping up with the times.