St. Augustine's works were more highly regarded within the Christian society. He was a bishop until his death. Libanius was more of a politician and scholar and yes, I believe that Libanius's works weren't held in quite as high regard because he held onto the Greek pagan ways, even though the times were changing.

One thing that Augustine and Libanius shared was the fact they both had concubines. But Augustine saw it as a relationship with lust and abandoned his concubine due to his status to be properly married to someone of equal status while Libanius was content to love his concubine until his death.(Brown, 51)

Brown, Peter, Augustine of Hippo: A Biography, Revised Edition with a New Epilogue