thus taking the religion of widows, slaves, and freedmen and making it the religion of kings.

Perhaps that's part of the materialist answer right there. It was the religion of the poor and destitute and slaves, so the some elites coopeted it to maintain/increase their political hegemony. If Christianity was a social movement, then the Roman elites taking over and neutering it's revolutionary character makes a lot of sense.

I'm thinking back to a great book I read, not Marxist as far I know, called "The Germanization of Christianity." The first third of the book is about the Romanization of Christianity, turning a religion of desert monks and beggars into a bureaucracy.