Well because Pokfulam, Aberdeen, Southside don't really have hotels in the area. If they do, these tend to be 4-5 star ones that are often fully-booked and not starving for biz or belong to international groups and are not interested in this biz.
Who knows once the MTR South Side line is completed in a few years time and more local hotels built, you may see some HGs in the South Side?
Actually I am surprised the one local hotel in Wong Chuk Hang doesn't have any listings as it would be a great service for the dads who have to accompany their families to Ocean Park.