Arcs,
Simply put, it's because fluorocarbon's composition is of denser materials, so it sinks. Mono is less dense than water, so it floats, fluorocarbon is denser than water, so it sinks.
Fluorocarbon's sinking properties creates a shallower degree of line bow, so when you set the hook the energy is transferred more efficiently to the lure, creating a more solid feeling and better set than mono. Most people say that fluorocarbon has less stretch than mono (pros included...), but that's generally incorrect. A few select fluorocarbons truly have less stretch than mono and that's because they're made of harder materials than the traditional castable fluorocarbons. There've been several studies regarding stretch and it's scientifically accurate that nearly all fluorocarbon line has as much stretch, if not more, than monofilament line. It can be verified by a little research and doing a little experiment at home... Sunline Shooter, a very popular fluorocarbon, is harder and more dense than the average fluorocarbon, so there's less stretch, making it a very good flipping/pitching line. Also, because of its hardness, it's also more abrasion resistant and resists water absorption longer.