Black represents a lack of light/colour - i.e. absence/unconsciousness/darkness - death has often been nicknamed the long sleep - i.e. your eyes are closed forever, and it's dark, i.e black. The skeleton represents death, and the dead. More specifically OUR death and OUR dead. Which has always been the focal point of human storytelling, and folklore, and even music. The robes and hood represent its hidden face, i.e. the idea we know very little about death, and more specifically what comes AFTER death. The scythe is a farming tool, used to harvest crops by slicing the produce from the stems - I think it was specifically designed for corn and wheat. Only in this case, the scythe is used to harvest US. Which is where the nickname "reaper of souls" or "grim reaper" comes from - to reap is to collect harvest. Death harvests our souls, to take them to whatever afterlife.

Because

A number of symbolisms. I’m not entirely sure where the scythe originated from, but it seems most likely Greek, as they had at least 3 gods of planting and harvest, and often compared other elements to them. The scythe was seen to “reap” souls just as it harvested crop. The cloak was prolly cuz he’s edgy.