The problem isn't free site, it's extensibility. Chess/chess.com has 100s of variants, ability to study games, and lots of other chess specific features.
Bga has fixed set of features for all its games. And their focus is divided.
Look, for all I know Gigamic is just waiting for someone to come along and make chess.com for Quoridor, and you're the guy to do it.
But I think your ignorance of the hobbyist game market is revealed a bit here already (not having full context for BGA, not knowing if Gigamic still owns it - they certainly do and had a booth at Essen this year like they do every year) - and you're vastly overestimating not just Quoridor's popularity but the size of the entire market for non-chess/go/tables/poker games. And I don't just mean financially - dozens of variants are no good if there are barely enough players to match dozens of games to begin with.
Did you try contacting BGA to see if you could get the Studio files for Quoridor?
Expanding on one point - not just market size, but the amount of competition. As you said upthread, there's no shortage of reasonably popular [1] modern abstract games, there's plenty of other options for people to play. It might be possible to try and promote Quoridor more broadly, given how simple its rules are, but that'd be tough (and marketing it more yourself, beyond just providing an implementation, would probably cause issues with Gigamic).
[1] Popular within the modern board game community, at least.
Not even “competition”. BGA is more or less allowed because it’s assumed it drives sales. A Quoridor-only site would probably sell Quoridor fine, but BGA will also sell Cuarto, or a dozen other Gigamic games. Or vice versa - BGA probably does more to draw e.g. Catan players into checking out Quoridor than a dedicated Quoridor site would.
Also BGA explicitly asks for permission to post these games.
Also BGA is pretty big and large. It's already one of the best places to put digital implementations of games, and that's why you see major hobby games come out on there, like Wingspan and Ark Nova, with about 75 or more new games added to the site every year, including games that haven't even been released yet and still on Kickstarter, like Undergrove recently.
And yeah, I've bought probably at least a dozen games now after playing them on BGA first. It's fun to play them on BGA, but I'd also like to play them in person with my friends, too.
Bga has fixed set of features for all its games. And their focus is divided.
This game atleast deserves its own home.