One big reason most companies prefer to hire locals is the whole hassle of getting visas for foreigners to work wherever. To get over this hurdle, you have to be offering something their local candidates don't have - languages, experience of a kind that is difficult to get locally (tough, I know, for entry level positions).

If you already have working privileges in Singapore (say, were born there or something) make sure they know that.

It's generally a royal PITA to hire a foreigner over a local person. You have to make it worth their while somehow.
Cheers,
Bev