There are two different cuts of beef short ribs. One is from the chuck (the shoulder), which is cut into rectangular chunks of meat, generally 2 to 3 inches long, and includes layers of fat, meat, bone, and connective tissue. The second is from the short plate (the underside of the chest), and generally includes five ribs (numbers 6 to 10), and which is meaty but also includes a lot of connective tissue. This cut is also known as plate short ribs or simply beef ribs.
As you can guess from the recurring use of that appetizing phrase "connective tissue," both cuts are tough, and require long cooking to soften them up. We do not know the history of how these particular cuts came to be singled out, but clearly they represent the determination to get the most value from the carcass once the better cuts of meat have been taken.