User blog comment:B214/Season 3 confirmed!!!!!!/@comment-24.104.16.162-20150329163637/@comment-23853100-20150401120552

Rias was probably named as the next heir because no one expected Millicas to be born so early, remember Devils have low birth rate, even Sirzechs and Rias has a 200+ gap in age, Lord Gremory and the other probably though that Rias would be 50-60+ before Millicas is born.