User blog comment:Psy0Kryst/Masai Kurosaki in Soul Society/@comment-7092747-20130603163603

As pointed out, a person killed by a Hollow does not go to the Soul Society as their soul is consumed and thus destroyed by the Hollow. This is why Hollows must be hunted down as the loss of souls in the World of the Living creates an imbalance of soul transference between the two worlds (as noted by Mayuri).

On top of this, the Rukongai is massive. Alone it is split into four compass-based sections (North, South, West, and East) and each of those four sections is split up further into eighty districts which means in total there is 320 districts in the Rukongai.

The Seireitei in itself takes three days at a walking pace to cross from one gate to its opposite. If the Seireitei is that large in size, you can easily begin to surmise just how massive in size the Rukongai is and come to understand that even if Masaki's soul had gone to the Soul Society why it would be impossible for Ichigo to find her.