I'd say that the major reason is cost.

Inside Australia you have the major peering fabrics which are on the order of a dollarish a mbit (cap). Compare that to $50-200/mbit for international transit and you'll see the problem. Quickly compare the cost of a gigE link.

About the only "international" player that doesn't (really) have this problem is NZ. Transit to NZ is ca $5/mbit last time i looked at it.