Commentary Purg II 93

Casella has evidently been dead some time (for at least slightly more than three months, as verse 98 will make plain) and Dante wonders why he has been so long between death and his first step toward salvation, arrival in purgatory.  Beginning with Poletto (DDP Poletto.Purg.II.94-99) and Moore (Moor.1896.1), p. 168, commentators have seen a connection here with Charon's unwillingness to take certain of the waiting shades across Acheron (they all are eager to be taken) and in fact picking and choosing among the waiting throng (Aen. VI.315-316).  We might continue the thought: In Dante's poem Charon takes all bound for hell at once; only those bound for purgatory need to be winnowed by the transporting angel, with some having to stay longer in the world, near Ostia, thus mirroring a sort of prepurgation that the poet has invented, offstage, as it were.