Because PLA is made from sugar, funny enough, if use PLA from Europe to 3D print it smells like cotton candy (cause made from cane sugars) whereas PLA in the US smells like popcorn (because made from corn syrup). It's also why its completely biodegradable, if you leave it outside, bacteria and UV light will eventually break it down, its also why its not a good material for anything exposed to environmental elements or something that needs to be durable for a long period of time. Its fine for most things, including biodegradable cups and straws. All that said, its probably the least toxic of the plastics considering its source.

PLA has been around for a while, its not new with 3D printers, its just commonly used for 3D printers because it has excellent properties for 3D printing.