The likes of Al Franken, Katie Hill, Anthony Weiner weren't impeached. They were pushed to resign. The same could have been done to Northam.
It's a lie by omission to imply that Democrats are just as upset at him now as they were when the news broke, and that the main reason he kept his job was the state constitution. The calls for Northam to resign diminished when it seemed clear that the next two Democrats in line were both tainted, AG Herring for something similar and Fairfax for something worse. It may also be that when the situation became more complicated than replacing a middle-aged white guy with an African American rising star, Democrats realized that they made a rush to judgment in determining that something stupid but legal a med student did in the 1980s should justify removing a person currently in office.
Northam is not currently treated as if he were a pariah, which is what you would expect if someone stays in office despite the consensus of the party that he is unfit to be Governor. Political figures who called for his resignation later appeared at events with him.
https://www.foxnews.com/politics/ter...ckface-scandal
It does appear that there was a consensus that Fairfax should not become Governor when it turned out one woman accused him of rape in college, and another accused him of sexual assault at the 2004 DNC. You could make the case that with Fairfax, Democrats clearly prefer him out of statewide office and are hindered by regulations about impeachment. There were still options available but they didn't want to choose that fight. For example, the Democratic Congressional Campaign Committee announced that they would refuse contracts with any political vendors that support a primary challenge to any Democratic member of Congress for any reason.
https://nymag.com/intelligencer/2019...democracy.html
The DCCC and any organization that funnels money to Democratic lawmakers could have made a similar announcement regarding anyone who worked for Fairfax. But it's more important to support incumbents than to encourage rapists to resign.