Belts must be quieter, easier to package and easier to change. Chains will be noisier, bulkier and be more fiddly to change.
Also chains need lubrication or they will wear out. They also need a beefed up tensioner to keep them tight.
A chain going west can do far more damage than a belt KIA and a loose, poorly tensioned chain will soon eat it's way out of your block.
What I want to know is why nobody does gear-driven cams, a la Honda VFR motorbike. Blinking indestrucible.
And possibly eye-wateringly expensive to make.