User blog comment:B214/High School DxD EX/@comment-201.239.133.207-20170104020556/@comment-9595671-20170104205732

I believe in Robertina's case that since Ravel married Issei and the whole thing with Rias being his main wife, Robertina couldn't inherit the Phenex name.

For Airi's case? I have no idea on that one, I'd say it's cause of being excommunicated and because she was raised by the church, but Xenovia's last name came from Griselda raising yet her son has it too.