A simple answer is, because Honorius was in Ravenna, which was his center for ruling, so the sacking of Rome by Alarich didn't affect him in a personal, direct way. Although this was the first sacking of Rome since the Gaul's in 395 BC and people really saw it as important event (as can be seen in the writings of Hieronymus), the effect for the emperor and the administration was relatively low, because they were in Ravenna, which Alarich couldn't capture (he tried).