In the work From the Maccabees to the Mishnah (Shaye J. D. Cohen, 2014) he states that the office of the Nasi ceased to exist around the year 425 CE (p.220), but does not explain why this is so. I would like to know the reason that the office of the Nasi ceased.
See this article on Wikipedia:
Gamaliel probably died in 425, as the Codex Theodosianus mentions an edict from the year 426, which transformed the patriarch's tax into an imperial tax after the death of the patriarch. Theodosius did not allow the appointment of a successor and in 429 terminated the Jewish patriarchate.