Its coz you always get all the Tangara's!! j/k
I too have noticed the lack of TFs in the north shore.
This is my theory. Suburbs that are on the "lower" social economic level e.g. more likely to be younger families with young kids will have more toys in general and suburbs that are on the "higher" social economic level e.g. less families with young kids will have less toys.
Otherwords, supply and demand. As much as I'd like to believe that the mainstream retailers cater to the more "mature" TF collectors, the fact of the matter is that its still directly aimed at mum and dads who have young kids.