Because units less than a litre are so common. 2ml of syrup, 4dl of flour, 12cl of wine, 0.5l of beer, etc. But things you measure by mass rarely go below 1 gram (in day-to-day life at home). Candy bar is 50g, a packet of cheese is 200g, a chunk of meat is a kilogram. Besides, most people just call it "a kilo" so the common units in the spoken language are "a kilo" when talking about mass and "a litre" when talking about volume