Rukia already explained that the souls on earth and in SS cycle back and forth in the natural order of things. Watch the episode where they explain quincies, and how their destruction of hollows as opposed to cleansing them disturbs the cycle.

A better question would be: Why did the parakeet kid REMEMBER what his mom looked like when everyone else forgot?
I think i heard that the whole parakeet kid thing was filler, though

ISSO: what are you talking about?