Because truly free games have no advertising budget, pretty much.
Honk
I first heard of Space Station 13 from a fanmade youtube ad that appeared in my recommended for some reason. Thinking it was a new indie game coming out, I clicked on it and it turned out to be really well done and made me want to try the game. Then I went on a bad server and I wouldn't touch the game again until someone on bay12 recommended I try goon instead.
Post before the Ninja:
Yeah, they rely on word of mouth to spread. Best way to get your free game some publicity would be to make it a relatively high quality specimen of a niche taste and post it to forums filled with a dedicated community. RPG maker games come to mind. Traditional roguelikes were also like that though I worry that the prevalence of rogue-
lites has made it basically impossible for new rogue
likes to get a big following. The biggest newcomer I can think of is Cogmind, but that thing's following only amounts to some occasional mentions in youtube comments (probably in the comment sections of other websites as well), as well as a thread on this forum. It is still in early access though. Maybe when it comes out it might be able to at least sit beside IVAN on the official roguelike pantheon.
I'm not dissing the dev btw, I'm just lamenting how times have changed and how you have to work so much harder now.