Background: Born into a poor family in Waco, North Carolina, Patterson was the youngest of eleven children and experienced an insular and troubled childhood. His family moved to Brooklyn, New York, where Floyd was a truant and petty thief. At age ten, he was sent to the Wiltwyck School for Boys, a reform school in upstate New York, which he credited with turning his life around. He stayed there for almost 2 years.
Context: After a series of defenses against fringe contenders (Hurricane Jackson, Pete Rademacher, Roy Harris, and Brian London), Patterson met Ingemar Johansson of Sweden, the number one contender, in the first of three fights. Johansson triumphed over Patterson on June 26, 1959, with the referee Ruby Goldstein stopping the fight in the third round after the Swede had knocked Patterson down seven times. Johansson became Sweden's first World Heavyweight Champion, thus becoming a national hero as the first European to defeat an American for the title since 1933.  Patterson knocked out Johansson in the fifth round of their rematch on June 20, 1960, to become the first man in history to regain the Undisputed World Heavyweight Championship. Johansson hit the canvas hard, seemingly out before he landed flat on his back. With glazed eyes, blood trickling from his mouth and his left foot quivering, he was counted out. Johansson lay unconscious for five minutes before he was helped onto a stool.  A third fight between them was held on March 13, 1961 and while Johansson put Patterson on the floor, Patterson retained his title by knockout in the sixth round to win the rubber match in which Patterson was decked twice and Johansson, once in the first round. Johansson had landed both right hands over Floyd's left jab. After getting up from the second knockdown, Floyd abandoned his jab and connected with a left hook that knocked down Johansson. After that, Patterson came on with a strong body attack that wore down Johansson. In the 6th round, Johansson caught Patterson with a solid right. But the power in Ingemar's punches was gone. Patterson won the fight in the 6th round by knockout.
Question: Why was the fight stopped?

Answer:
the Swede had knocked Patterson down seven times.