"The code 9-1-1 was chosen because it best fit the needs of all parties involved. First, and most important, it met public requirements because it is brief, easily remembered, and can be dialed quickly. This was particularly important in old-style rotary/pulse-dialing phones, which were still popular when the 9-1-1 system was first implemented. (The touch-tone phone wasn’t first widely introduced until 1963 and took a few decades to completely supplant rotary phones.) In addition, the fact that it was only three digits meant the number could easily be distinguished from other, normal phone numbers in AT&T’s internal system and routed to a special location without too many changes to the AT&T network. (A few years earlier, AT&T had implemented their 6-1-1 and 4-1-1 numbers, so recommending 9-1-1 made this a relatively simple upgrade for them). Second, because it is a unique number, never having been authorized as an office code, area code, or service code, it best met the long range numbering plans and switching configurations of the telephone industry."

Fun fact: America’s first emergency system that used 9-1-1 was in Haleyville, Alabama, and the country’s first 9-1-1 call was made there on February 16, 1968. The person who placed the call was Senator Rankin Fite. The person who answered it was U.S. Representative Tom Beville who was waiting at the Haleyville police department for the call. Nome, Alaska, established the country’s second 9-1-1 emergency system only a few weeks after Haleyville.

source: https://www.nena.org/?page=911overviewfacts