I partially blame Kickstarter. Kickstarter makes it relatively easy to launch a new game and find people who think "Yeah, that sounds cool, I want me some of that." A lot of the games sold will find their way to someone's bookshelf and never hit the table – particularly if there's a lengthy delay between crowdfunding and fulfillment (because by then the customer will have found a new shiny). But any supplements for that game will have a diminished audience, because you're selling those to the people who bought the original game and likely only the ones actually playing it. To take an anecdotal example, the Troubleshooters original Kickstarter made about $80k from 1,116 backers. A few years later they Kickstarted two adventures for the game, and got about $25k from 335 backers – so less than 1 in 3 followed up. And then you as a designer (or at least someone running a game publishing business) have to ask yourself: do I want to make a sourcebook or adventure for the game I released last year, or do I want to make a new game that will get me three times the money for the same effort?
That said, it's not impossible to make a game with a deep bench. It's just a lot harder. Modiphius has had some success with kickstarting entire game lines at once, though I think all of those have been licensed games (e.g. Infinity, Conan, or Mutant Chronicles). Some of the sub-lines of Onyx Path's Trinity Continuum have gotten quite a few supplements, mainly Aeon and Aberrant – but where White Wolf once would release a series of 8 sourcebooks with each covering one psi order plus the geographical region where they focus, Onyx Path did one sourcebook for all the psi orders and another for the Earth and the Moon.
The main problem here is, I think, that publishers haven't fully internalized that sourcebooks won't be a thing, and still write core books as if they are. You get core books that lightly touch on various aspects of the rules and world, whetting your appetite for more, but then that "more" never comes. That's where I think PEG's model is most successful: they do a setting book and a plot point campaign with maybe some extra bling (maps, cards, etc.), and then move on to the next, while still having the same rules. For example, I'm currently having a lot of fun running the Degrees of Horror campaign for the East Texas University setting in Savage Worlds. But once I've done that campaign? I don't really feel the need to go back for more. Maybe I'll do Transcendence for the Han Cluster setting after that, or Horror at Headstone Hill for Deadlands.
