Why were ousia and hypostasis synonymous in the Nicene Creed?

Why were ousia and hypostasis synonymous in the Nicene Creed? ---- In the original 325 A.D. Nicene Creed, an anathema is included which has ousia and hypostasis as synonymous. In this case, the Trinity is one hypostasis ( = homoousios). >And in the Holy Spirit. But as for those who say, There was when He was not, and, Before being born He was not, and that He came into existence out of nothing, or who assert that the Son of God is of a ***different hypostasis or substance*** (ὑποστάσεως ἢ οὐσιάς) or created, or is subject to alteration or change these the Catholic and apostolic Church anathematizes. Source: https://earlychurchtexts.com/public/creed_of_nicaea_325.htm It seems also the meaning of υποστασις in Hebrews 1:3. >He is the radiance of the glory of God and the exact imprint of his ***nature*** (υποστασις) (ESV). The ASV has "substance". However, in later centuries hypostasis began referring to the "person", not the "nature" or "being" of the Trinity. **Why did such change in definition occur?** It would be helpful to address the semantical development of υποστασις on how it changed from "substance" (nature/essence) to "person". >The Church confesses is that God is three Persons (hypostasis) in one Essence (ousia). Source: https://www.google.com.ph/amp/s/exploringthedepthsofthedivine.wordpress.com/2015/08/12/god-as-trinity-orthodox-trinitarianism/amp/