Because Singapore incinerates waste, biodegradable plastics are not worth the extra expense: they will simply get incinerated along with regular plastic. 

And no there is no meaningful difference in the toxins released during incineration between regular and biodegradable plastics. Unlike open pit burning, the incineration process in Singapore traps toxins.

Likewise, it is not possible to compost most biodegradable plastics at home. Most biodegradable plastics need high heat industrial composting to break down. 

That also means if you throw these plastics into the ocean, they will not magically disintegrate: a biodegradable straw in the ocean is as dangerous to marine life as one made of regular plastic.