"Descensus Christi ad Inferos"

"The descent of Christ into Hell."

Canto IV of the Inferno, Virgil relates to Dante that Christ(in the time between his death and resurrection) brought salvation to Adam, Noah, Moses, Abraham, David, Jacob(Israel), Rachel, and other Old Testament figures, but not to the Greek and Latin poets, philosophers, and certain Muslims: Saladin, Averroes, Avicenna, who are all still in Limbo when Dante is there.

Why are these exemplary Greek, Latin, and Arabic souls left in Limbo? Doesn't add up to me.