Per Wikipedia https://en.wikipedia.org/wiki/List_of_North_American_Numberi...
"The syntax rules for area codes do not permit the digits 0 and 1 in the leading position."
My guess would be it's to avoid ambiguity with the fact that 1 is also the country code. If I recall correctly, historically, dialing the 1 was necessary for any long distance call (even if not international).