I think that’s a good question.
In my opinion the problem is the translation ‘unconditioned’. Asankhata literally means “without (what is) created”. The word sankhata is used with reference to a raft that is put together, for example, and we don’t say a raft is “conditioned”. (Snp1.2) So Thanissaro for example translates sankhata as ‘fabricated’, which I think is better than ‘conditioned’. The prefix a- in asankhata functions as a prefix of absence, not of opposite. As the Critical Pali Dictionary says for the prefix: “In adj. comp. with subst. = ‘without that’,” (Other dictionaries say similar things, but this is the one I could find quickest.) So asankhata refers to the absence of anything created.
Nibbana can refer either to the cessation of the defilements or the cessation of existence at death of an enlightened being. I would say both are conditioned, in the English sense of the word, because they depend on the eightfold path. So they aren’t unconditioned in the sense that we normally understand this term.
Nibbana is a metaphor for cessation (the metaphor of a fire going out), and cessation is not a thing that exists forever and we just reach it, as some people argue in favor of ‘unconditioned’. The cessation of suffering, just as the extinguishment of a fire, is something that has a clear cause.