They aren’t scared people wouldn’t understand MMR. If they publish a number, and nobody understands it, they are no worse off.
They know that some of their playerbase knows game design. Some of their playerbase knows statistics. Some of their playerbase will put the pieces together and be able to conclusively prove whatever questionable things are going on if they reveal it. They’re scared that people will understand it.
A reminder that I wrote a thorough hypothesis on how and why we see the results we do, if you’re interested you can read it here: