Soviet television authorities announced Thursday they will not buy the ABC miniseries "Amerika"--concerning a Soviet takeover of the United States--but today they will broadcast "The Day After," a TV movie outlining the aftermath of a limited nuclear exchange, according to the Soviet news agency Novosti. The reason? Ratings. Novosti quoted an official of the Soviet television-radio agency Gosteleradio as saying that "Amerika" was rejected because "the majority of TV viewers in the U.S.A. could stand no more than the first few parts." "The Day After," on the other hand, "was watched by 100 million Americans . . . and is expected to draw millions of Soviet viewers," according to the unnamed Gosteleradio official.