The life of Maffeo Rex continues to have an outsized impact on the Rex Kingdom long after his mortal body had left Pacem.
After successfully claiming the crown King Maffeo set about constructing the Rex Kingdom from scratch.
When the Grand King finally claimed the throne in the Citadel there was a national holiday. Humanity had been wandering the world looking for purpose and suddenly this man appears who assures them he can guide them to glory. It must have been quite something to have been there. You can clearly see his influence in the number of babies now named Maffeo.
Though it was assumed to be common knowledge for us Rexiots when we started attempting to make diplomatic missions we needed a formal capital for them to be hosted at. With the vast majority of the surviving people of original Earth living there, it just made sense to make it a formal capital for the kingdom.