Here are a few more details about high-definition television’s evolution which we had to cut from Television: Critical Methods and Applications:
The Advanced Television Systems Committee (ATSC) was formed in the early 1990s to set a standard for HDTV—much as the NTSC had set the standards for the North American TV system decades before. In 1996, the FCC accepted ATSC recommendations for digital television (DTV), of which HDTV is one part. The conversion from analog television to DTV proceeded slowly and erratically. Wrangling over multicasting, enhanced TVand other issues delayed its implementation. Even so, the FCC decreed that analog NTSC television be eliminated entirely and replaced with some form of digital television. After many delays, the digital switch was finally flipped on June 12, 2009.
Much more information is available on the Wikipedia article titled, “Digital television transition in the United States,” including this frame capture showing a warning of the impending analog shutdown.