Debate moderators are generally supposed to be journalists. Their personal credibility may be, and usually is, low, Crowley and Harwood come to mind. Not to mention Ifil. But there's at least some cover there.
Rachel Maddow is not a news figure. At least she's in the same category as Bill O'Reilly, Tucker Carlson, Glenn Beck or Sean Hannity. She's a vocal advocate for a particular point of view. Her broadcasts are editorials. Very vocal ones.
More accurately, she spins conspiracy theories for people who believe that Trump has a red line to Moscow and that every foreign diplomat who stays in a Trump hotel is bribing him. (In all fairness, it's hard to distinguish crazy conspiracy theories from mainstream media coverage anymore.)
The DNC turned up its nose at FOX News hosting a debate, insisting that people like Wallace and Baier weren't journalists. Instead the DNC picked Maddow, a conspiracy theorist who is not on the news side, to host its debate.
That sends a message. And the message is that the DNC isn't interested in journalism, but in spouting partisan conspiracy theories.
And any supposed journalist who participates alongside Maddow should take a reputational hit.