Why is “nibbana” considered to be “unconditioned”?
Good question. Nirvana/Nibbana is not a state of mind, rather it's a quality or an aspect that is always present regardless of any conditions, but even though it's always present it is not always seen. The state of being aware of the ever-present N is dependent on conditions while the N itself is not.
You know this quality of mind that watches everything silently and just witnesses things without judging, whether circumstances are good or bad and whether you're well or tired or scared? N is similar because it doesn't stop nor change regardless of circumstances, not even when you're asleep or dead. This is why it's called unconditional.
Traditionally they say there are three ways to approach seeing N. One is through methodical incremental cessation of progressively more and more subtle forms of conflict inherent to craving/aversion, even of conflict inherent to aversion towards samsara and craving for N. Once you completely stopped generating conflict, completely gave up all aversion and craving - what's left to be experienced is N, pure and simple.
Another method is a bit more technical, it involves tracing how in your mind things are always defined relative to one another by dividing some sort of Whole into e.g. left and right, this and that, object and background, better and worse etc. In this method we try to ascend this graph of relative definitions up to its root by climbing the ladder of progressively more and more subtle forms of division that generates such relative concepts. The root of the graph is N which is not defined relative to anything else.
The third and final method consists in methodically trying to find something unalienable that truly belongs to oneself and cannot be taken away by the unstoppable passage of time nor swayed by circumstances. In this method one analyzes and discards all things that are subject to change and cessation, starting from obvious ones such as the physical body and moving on to subtle things such as one's memories, sense of identity, and various states of mind. As one fully and exhaustively understands the workings of conditionality and conditionally dependent arising/cessation of phenomena, one arrives at the only remaining answer, which is N.
From these explanations it should be clear that N is not something limited that is defined in juxtaposition to anything else and this unbounded supraconceptual suchness is exactly what allows it to not be dependent on any conditions or circumstances.