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[Adult Swim likely chose the 8-9 hour slot intentionally]
Family Viewing Hour once a policy at 8-9pm is generally still considered common decency to reserve family programming for the 8-9 hour
The Family Viewing Hour was a policy established by the Federal Communications Commission (FCC) in the United States in 1975. Under the policy, each television network in the U.S. bore a responsibility to air "family-friendly" programming during the first hour of the prime-time lineup (8 to 9 p.m. Eastern Time). The policy was abandoned in 1977 following a 1976 ruling by a federal court that found it violative of the First Amendment. However, the concept has continued on a voluntary basis in various manifestations.
https://en.wikipedia.org/wiki/Family_Viewing_Hour