Why is this tense used in this case?

Here is an excerpt from some Spanish literature: A pesar de que existen muchas versiones de los origenes del mole poblano, la mas común dice que *fue inventado* en el siglo XVII. Firstly why is the text highlighted in bold using the past progressive from. Secondly, why is SER being used in place of ESTAR. Thirdly, given that this is a completed event that only occurred once why is the preterite form not used. Thxs in Adv Nev

_fue inventado_ means _was invented_. This _is_ the preterite form. The thing is that it is in the passive. Here: He invented - él inventó - Preterite active He was invented - él fue inventado - Preterite passive Past progressive: He was inventing - él estaba inventando He was being invented - I think such a thing isn't even used in Spanish I hope that's clear now.