Hiya!
If what WotC said in the recent article about how they have 12-15 million players in NA is true, then they are way more massive then what 3rd party success would lead one too believe. 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. After all, when you have a playerbase of 12-15 million, those 100-500 backers on Kickstarter suddenly don't seem like so many. Even if each backer represents a full party of 5 eagers players, that's still less than 1% of the D&D market.
Thoughts?
Simply put, "Rulings...not Rules". That's the name of the game. That's what, imnsho, "caused" 5e to be so successful; the OSR was in full swing (still is to some degree) and people were just sick of rules, rules, rules, and more rules. Complete Book of This, Ultimate Guide to That, Billy-Joe Bobs Wondrously Weird Wombats, the books just kept coming and coming. This was..."bloat". Specifically, "Rules Bloat". The "old farts" (re: people like me who still play 1e AD&D), and an increasing amount of new folks brought into RPG'ing by us "old farts". What 5e did was basically say "Nope. Not doing that again. Maybe one or two books, tops, per year. Core rules will be simple and streamlined. Feats, Multiclassing, and many other things will be OPTIONAL. KISS principle will be in effect. Enjoy!".
...and that's why there is simply not a lot of folks wanting, or should I say "needing", a new book of options ever month. The 5e rules outright FORCES a DM to make rulings and interpretations when he/she is DM'ing. This does something that 3.x/PF never did...it builds a DM's confidence in their craft AND gives the Players confidence in the DM's decisions. This is the same sort of mentality that pretty much ALL of the OSR-style games did; put the DM in the role of Final Decision Maker, and not the latest rule book expansion that a player whips out at the table one evening.
IMNSHO, again, I think that the majority of 5e DM's are just enjoying the heck outta creating their OWN stuff for their OWN campaign world (wasn't it like, 25% or so of 5e DM's us FR...and over 50% use their own home-brewed campaign setting?...something like that anyway). Ergo, no demand for a lot of 5e "stuff". Which is a VERY GOOD THING indeed!
^_^
Paul L. Ming