Because everyone that knows about the existence of this prison knows that he is the one that designed the idea for Tartarus, they were worried that he would find a way to break out of the cell and in turn break away from the prison
So, the cell he is right now is not connected to the wall of Tartarus.
What most people don't know about Tartarus is that Tartarus itself is a living dimension.
It might not be as smart as human but it has instinct and it has some kind of sentient thought once in a while
The people imprisoning Boris in this prison feared that Boris might know how to manipulate the prison walls and so, a cell was crafted for him and then they relocated every other prisoner in the northern sector to other sector making him unable to spoke with anyone
He has been left alone for months. Boris understood why. They could not kill him and they fear the consequences of torturing him if one day he were to be released
