Pirahã is a very weird case, in general, as the language seems to lack a lot of things that one would consider essential. Last I heard, the best guess was that the tribe went through a bottleneck very recently, where almost all of the adults were killed, and all that were left were partly-educated children. In other words, the language used to contain all of the normal language complexity, but nobody alive learned it, and they haven’t had enough time yet to develop new complexity.