Well, according to trusted sources like Wikipedia, it is because we've moved away from the typewriter that single-spacing is becoming more common. With typewriters, inconsistencies in spacing made it so that you needed to have the double-space just in case, or you risk letters jamming together and shit. With computers, that shit automatically is spaced a certain distance apart. So it's because of the automation that people say we don't need the double anymore.

But I do anyways.