That is due to technical limitations in the telephone network that existed in the early days. Details here:

https://en.wikipedia.org/wiki/North_American_Numbering_Plan

But the short answer is that in the earlier days of the telephone network, after human operators but before internet telephony (roughly the 1AESS to the 5ESS era of phone switches) electromechanical telephone switches in the telco central offices stepped through the numbers in order, physically closing switches that connected an incoming line through successive layers of numbers until it made a final connection to the destination line. These switches were very “dumb” compared to what we have today, and so certain numbers had to be reserved to make sure that the first connection was routed to the right place.

“0” was reserved for special purposes, like Operator

“1” was reserved to initiate a long-distance call (indicating that it should connect to a long-lines switching center and the next 3 digits would be an area code)

“2” through (some higher number) indicated an area code or exchange (exchange is the next 3 digits after the area code and once directed the connection to a specific local switching center). Note that certain higher numbers like “8” were never used for area codes until technology advanced sufficently such that computers were doing this number parsing.

Certain other numbers were reserved. Like “555” which was set aside never to be used for reasons related to why certain phone numbers got swamped when they were randomly used in song lyrics :-)

By the time the emergency system was designed, the telephone system had advanced to the “IN” (Intelligent Network) and “AIN” (Advanced Intelligent Network) era of technology. One of the defining features of this era was that a computer was attached to the phone switch and the switch was smart enough to recognize when to hand off processing of the call to that computer for more sophisticated handling. These technologies allowed certain features we take for granted nowadays. Particularly toll-free (800 number) calling and routing long distance and billing through alternative carriers (remember MCI?). At this point, only “9” was easily available to denote the start of an emergency call AND was unique enough to be remembered as the special number. Plus, “touch tone” phones had become prevalent by then although rotary phones were still common. But the trade-off in dialing time for rotary phones was deemed necessary and acceptable.

[EDITS: proofreading and spelling]