Maybe because it's being offered by a third party.
Or maybe it's because there are near zero such parties. Any reasonable Bitcoin user who wants to be a coordinator runs JoinMarket, because it's decentralized and doesn't come with default coordinators which clarify that they fund chain analysis with coinjoin fees, and which might not accept you in the round if you're "naughty".
It only takes two clicks to access the config file. If there was an alternative coordinator you wanted to use it would only take a few seconds of effort to switch from the default one.
It only takes zero clicks to have your coins coinjoined with JoinMarket with various takers, as the software automatically switches to another taker in case one denies to have you in your round.