Personally, in the terms of training and professionalism, I would think that local models are a bit lacking. But maybe it isn't their fault. Maybe it's simply because that there isn't enough good modelling agencies with the experience and money to offer a complete training. And of course I guess it will be easier to hire foreigner talents than to actually train people from scratch. And I don't think being a model is such a desirable job amongst Singaporeans yet.
I don't agree that Caucasians and Eurasians always make good models. Take a look at the Japanese market. They worship almost anything that's Western, but when you look at their magazines, catalogues, you will hardly find any foreign models at all. In Korea, the local modelling industry is a multi-billion business, but it's largely boosted by their large creative and moderately active fashion industries. Look at Taiwan, these days they have plenty of really good models that have been dominating their fashion and creative scene. Lin Zhiling, Patina Lin, Janel Cai etc. Just to name a few.
There are many reasons, it could simply be that our local industries aren't developed enough to support an active modelling industry.