Because they are easier to work with. They serve as more functional units, kind of like building blocks that you can build things with. Hydrocarbons, which are compounds that only have hydrogen and carbon in their molecular makeup, that are longer in chain length are harder to work with. It is the same problem chemists run into when working with elements on the periodic table that have larger nuclear masses, they are too big, too unstable, more difficult in attaining results. Shorter chain hydrocarbons are simply more versatile, being able to bond in different ways. They are also more immiscible in water, making working with water an easier task. Shorter hydrocarbons would be methane, CH4, ethane, C2H6, propane, C3H8, and butane, C4H10. Anything past butane starts losing the characteristics described earlier in terms of ease and versatility of use.