There is a few reasons for this... The GTi was already sold in 2005 in SA where the ST only started selling Q2 2006.
The GTi has got a heritage so more people know about the GTi than people know about ST's hence people will buy GTi because they are uninformed consumers... go talk to women (even a lot of men) especially, how many know of the various cars in the same class?
I believe if the GTi was out a year before the ST and if consumers were more informed about other hot hatches in the same class, then there wouldn't be so many GTi's on the road but a better balance of the various models...
Plus go look in the UK... for every GTi there is about 3 ST's