Aoskars portfolio was portals of which there are tons in sigil. My guess is there is some feedback from having one of the things you directly reside over surroinding you that creates a feedback loop, like how most divine realms reflect the characteristics of their gods. Aoskars belonged in sigil because it was similar to what his divine realm would have been. Power flowed between them and he was integrated.

The amount of death and such Vecna needed was not a part of the landscape so he tried to make it so and it did not go well.