Because from what I see when the heavenly principles started becoming more contr​oling they starts devolving dragons to better suit humans and even kill many of them.

I think there duty is just preservation of teyvat not the species as if they were kins they should have done something but they did not.

Mostly I believe this is because of their detachment as the moons always stays on the moon so they don't have much attachment to dragons as there is not much interaction on the other hand the moon sisters are very close to angels and other moon emissaries as they interact with each other and even phanes more than dragons

I don't really like this tbh because I thought they should have done something though there is still some information that is missing so I will hold my opinion for now.
Are the moon sisters really draconic?