Why do Harrison Bergeron's parents not respond with more feeling to what they have seen on their television set?

Hazel does not realize what has happened because she is intrinsically mentally dull, while George does not fully grasp the situation on the television because his initial recognition of Harrison was electronically removed from his brain. Also, when his son is shot, he is out of the room.

When the announcement is made on the television program that Harrison Bergeron has just escaped from jail, a photograph of their son appears on the television screen, seeming to jump up and down "as though dancing to an earthquake." George exclaims, "That must be Harrison!" because Harrison created the same "earthquake" in their home when he was living with them. This thought is instantly met with the crashing sound of a car collision in George's head. This electronic sound removes George's thought.

Further, when Harrison and the gorgeous ballerina, who have removed their handicaps, leap together in the air, Handicapper General Diana Moon Glampers fires a double-barreled shotgun that instantly kills them. At that precise moment, the Bergerons' television burns out. 

Hazel turned to comment about the blackout to George. But George had gone out into the kitchen for a can of beer.

As he returns, George has a handicap signal shake him up. He then notices that Hazel has been crying, but she cannot remember why.

"Something real sad on television."
"What was it?" he said.
"It's all kind of mixed up in my mind," said Hazel.
"That's my girl," said George. He winced. There was the sound of a riveting gun in his head.

Hazel and George are desensitized by technology. While Hazel's mental capacity is merely that of a passive receptor, George does have independent thoughts, but they are quickly removed because of the noisy thought-killing mental-handicap radio in his ear that he is required by law to wear.

With this characterization, author Kurt Vonnegut clearly wishes to alert readers to the desensitizing effects of technology that manipulates people's minds from having thought-provoking ideas to thought-shifting ones.