* Street food is very tasty, but often laden with gular (palm sugar) or rock sugar (case in point: Sambal). You don't notice because it's spicy!
* Sweet drinks (milo, teh tarik) often accompany the spicy food.
* The street food is so cheap, and available pretty much everywhere. People avoid cooking at home[1]. Fresh fruit and vegetables are not that cheap.
* Because the weather is so hot and humid, people avoid walking.
* People like to eat white rice, as opposed to brown rice (high GI)
* Society has transitioned to office work rather than manual work
* People can afford larger portions because they are wealthier
* The weather is hot and humid, so people avoid walking outside during the day! AC office to AC car to AC shopping mall, etc, etc.
[1] One fascinating thing I have noticed is the concept of a "wet" and "dry" kitchen. The dry kitchen is inside the house, and is almost a show-piece. The wet kitchen is where the real action happens (albeit by the maid), but is typically a lean-to adjoining the back of the house.