Maybe because it's being offered by a third party.It's none of their business how much volume there is on third party coordinators. Anybody could be running a private coordinator that isn't advertised publicly and Wasabi would have no idea how much volume there is because they are not involved in any way.
So if the third party coordinators are not publicly accessible, how is the average Joe going to use them? It would also be trivial for Wasabi to answer this question - just pull every Wasabi coinjoin from the blockchain, and then subtract their own ones. The fact that they don't tells you everything you need to know.
He can't tout "Use a third party coordinator" as the ultimately solution for every problem Wasabi has, when he knows fine well there are very few third party coordinators and the ones that exist have almost no volume.
And none of that does anything to solve the rampant address reuse, some coinjoin participants receiving zero privacy, or Wasabi grinding outputs in to dust as has been discussed elsewhere.