Why is “nibbana” considered to be “unconditioned”?

As stated in the other answers, Nibbana is not a state of mind or thought of the mind or state of consciousness, but rather, it's something unconditioned that is experienced by the mind. You can find more details in this answer.

Also the sutta quote below is helpful.

From AN 9.34 (translated by Ven. Sujato):

Ven: Sariputta: “Reverends, extinguishment (Nibbana) is bliss!
“sukhamidaṃ, āvuso, nibbānaṃ.

Ven. Udayi: “But Reverend Sāriputta, what’s blissful about it, since nothing is felt?”
"kiṃ panettha, āvuso sāriputta, sukhaṃ yadettha natthi vedayitan”ti?

Ven. Sariputta: “The fact that nothing is felt is precisely what’s blissful about it.
“Etadeva khvettha, āvuso, sukhaṃ yadettha natthi vedayitaṃ.

In Buddhism there are two elements: conditioned and unconditioned, per MN 115.

There are these two elements: Dve imā, ānanda, dhātuyo—

the conditioned element and the unconditioned element. saṅkhatādhātu, asaṅkhatādhātu.

When a mendicant knows and sees these two elements, Imā kho, ānanda, dve dhātuyo yato jānāti passati—

they’re qualified to be called ‘skilled in the elements’. ettāvatāpi kho, ānanda, ‘dhātukusalo bhikkhū’ti alaṃvacanāyā ti.

Nibbana is the unconditioned element or sense object (ayatana) that the mind can sense, as follows:

There is that sphere (ayatana), monks, where there is no earth, no water, no fire, no air, no sphere of infinite space, no sphere of infinite consciousness, no sphere of nothingness, no sphere of neither perception nor non-perception, no this world, no world beyond, neither Moon nor Sun. There, monks, I say there is surely no coming, no going, no persisting, no passing away, no rebirth It is quite without support, unmoving, without an object,—just this is the end of suffering.

Ud 8.1

If we don't know what "ayatana" means, MN 148 explains:

There are the sense fields of sights, sounds, smells, tastes, touches and (other) phenomena (aka "only-mind-sensed-objects").

Rūpāyatanaṃ, saddāyatanaṃ, gandhāyatanaṃ, rasāyatanaṃ, phoṭṭhabbāyatanaṃ, dhammāyatanaṃ

MN 148

For example, a tree or rock are physical objects (rupa-dhamma) and are not mental objects (nama-dhamma) yet the mind can sense these physical objects via the eye sense organ.

Similarly, Nibbana is an type of element that is not physical or mental yet the mind can sense it.

For newbies, it can be said Nibbana is a peaceful quiet stillness, such as when there is no noise and no wind. Such a peaceful quiet stillness is not something physical, nor is it mental. Yet this quiet peaceful stillness can be experienced when the mind is free from defilements. The following may help:

Suppose, bhikkhus, there was a house or a hall with a peaked roof, with windows on the northern, southern, and eastern sides. When the sun rises and a beam of light enters through a window, where would it become established?”

“On the western wall, venerable sir.”

“If there were no western wall, where would it become established?”

“On the earth, venerable sir.”

“If there were no earth, where would it become established?”

“On the water, venerable sir.”

“If there were no water, where would it become established?”

“It would not become established anywhere, venerable sir.”

SN 12.64

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.