The teaching of original nature (benxing 本性) is not the same as the teaching of nirvana. First, nirvana is one of the core teachings of Buddhism while original nature is itself a Chinese term we can connect to the teaching of tathagatagarbha, and tathagatagarbha is something we find only in specific Mahayana sutras. Understandably, non-Mahayana schools never even had the concept of original nature and even in Mahayana it was something only a few traditions took up. And here is the second reason, that original nature doesn't necessarily fit into the teachings of a tradition. For instance, both Madhyamaka and Yogacara - the two main systems of Mahayana - has teachings contradicting the idea of an original nature. Or, even when in some cases teachers of those schools accepted tathagatagarbha, they came up with their own interpretation of it. It was in Chinese Buddhism that because of these reasons they actually said that besides Madhyamaka and Yogacara there is a third major Mahayana thought, Tathagatagarbhavada, i.e. those who take original nature as a fundamental teaching.
"While teachers of the middle way, mind only, transcendent wisdom, mantra, and other schools may have their own assertions, the fulfillment of those intentions is the same. There is not a single thing that is not contained within mind."
(Gampopa to Düsum Khyenpa, in "The First Karmapa", KTD Pub, p254)
“If you recognize the world of appearance and existence as the mind, realize the mind itself as empty, and have no grasping at the superiority of your realizations — this is the ultimate view."
(Chegom Dzongpa, in "The Book of Kadam", Wisdom Pub, p609)