Is it because dubov is Russian? I haven’t been up to speed on these things.
Ian Nepomniachtchi: “We should thank Dubov!”

Is it because dubov is Russian? I haven’t been up to speed on these things.
That is the reason in a nutshell.
The idea that a member of the support team of a professional should quit their job because the boss is going to face a countryman of them is clearly ridiculous. Imagine that would happen in tennis.
Probably Carlsen and Dubov made a contract in 2018 with the long term goal of defending the title. Quitting that because the challenger is from Russia would be close to immoral.
https://chess24.com/en/read/news/ian-nepomniachtchi-we-should-thank-dubov
Ian Nepomniachtchi has responded to the controversy over Daniil Dubov helping Magnus Carlsen for the World Chess Championship match. The challenger doesn't show the same anger as others have on his behalf, but does state that it would have been "more logical" for Daniil to "remain neutral". On the other hand, he points out that the games where Daniil's influence were clearest were the ones where he had the best chance of picking up a win. Ian also talks in depth about the blunder in Game 8, which all but sealed his match fate.