Because fat starts with a series of three carbons, then three oxygens, then 6 or more carbons.Off of those carbons are hydrogens. If all carbons have two hydrogens, the carbon tails are straight and can be stacked neatly. This is a fat and is saturated (dense). If some hydrogens are 'missing,' then the carbon tail will bend towards the empty space. Now it cannot be stacked so well, and will not pack. It will remain a liquid at room temperature.