User blog comment:B214/Volume 22 spoilers/@comment-213.120.234.106-20160720102919/@comment-9595671-20160721035555

I always thought their birth rates weren't as much a lower chance in the sense of conceiving, but because they just didn't have children that often due to their lifespans. Even if that is the case, I'm not sure how reincarnation would effect since they end up becoming Devils... that's a tough one.

It could be possible that Rias' father finally retired around this point, and Rias just had too much to focus on with becoming the new household head to have a kid with Issei immediately. As a result, Issei had a child with Akeno and Asia first and then Rias once she had more free time to raise a child.