^ Because you are stopping two notes at once. You are playing on two strings simultaneously. You can see clearly why they are called what they're called. To "stop" a string on a string instrument means to hold it down with the left hand to produce the corresponding pitch (as opposed to the open string in which no finger of the left hand holds down the string). Double-stops are practiced by violin, viola, cello, and bass players for intonation and technical purposes that serve both the left and right hands. Triple-stops (so, three notes played at once) are pretty much the greatest extent to which a string instrument can bow more than one string at one time. A note value that calls for four pitches played at once, a quadruple stop, would need to be "rolled" or "broken" as we say in string player terminology since the bow cannot hold down and play all four strings at once.