Artificial sweeteners found in diet drinks are generally more powerful sweetening agents than sugar, much more so. Hence, the required concentration of, say, aspartame would be much lower than the equivalent concentration of sugar required to sweeten the pop. The high concentration of sugar in Original Coke makes it more dense (heavy) than water so it sinks, while the very low concentration of aspartame in diet coke makes it similar to water in density so it does not sink. By the way, this also makes diet coke freeze at a higher temperature than Original Coke because it is the concentration of the solute and not the nature of that compound that lowers the freezing point of water.