Because, given that Nightshade is already on Wismuth, staying there without the ability to draw on the Void would be tantamount to suicide.

[She says it quite plainly.]

And, since you refuse to be sentimental yourself, I will say that it's because there are people here that care about you - and if you found a solution for Strange and Steven's problems, we can find a solution to yours. You're an excellent Acquisitioner besides that, and a good man.

You probably deserve better than the things you've been dealt... so far as a fae can judge anything about moral fiber.

[She winks.]