I'm guessing it was the smallest practical size to encode alphanumeric data, and making it bigger than it needed to be would have added mechanical complexity and expense.
https://en.wikipedia.org/wiki/Six-bit_character_code: "Six bits can only encode 64 distinct characters, so these codes generally include only the upper-case letters, the numerals, some punctuation characters, and sometimes control characters."
I'm guessing it was the smallest practical size to encode alphanumeric data, and making it bigger than it needed to be would have added mechanical complexity and expense.
https://en.wikipedia.org/wiki/Six-bit_character_code: "Six bits can only encode 64 distinct characters, so these codes generally include only the upper-case letters, the numerals, some punctuation characters, and sometimes control characters."
There was apparently a 5-bit code in use since the 1870s, but that was only enough for alphas: https://en.wikipedia.org/wiki/Baudot_code