
1A2 died a slow death, starting with the growing popularity of "electronic" key phone systems in the early 1970s, and ending around 2006 when Cortelco stopped making the ubiquitous 2564 six-button phone.
Here are some of the factors that helped electronic phone systems to kill 1A2:
Simpler, faster, easier installation with fewer wiresNew features such as memory dialing, voicemail, computer programming and Caller IDCompetition from new manufacturersIntegration of cordless phones into systemsSmaller phones that could handle many linesFewer mechanical parts that could wear outSystem features -- such as which lines ring at which phones -- could be easily set up and modified by programming, rather than by wiring changesPhone displaysSmaller KSUs that use less powerDecline of electronic manufacturing in the USA and growth in AsiaEnd of government-endorsed Bell System monopoly in providing phone equipmentEnd of Western Electric manufacturing in the USAIncreasing number of computer people involved in corporate telecommunications departments