Why is 1 not a Prime Number?

Is 1 a Prime Number?

Friends, in one of the post where I have described ‘Prime’ and ‘Composite’ Numbers, one of the curious visitor have asked me a very logical question. I will quote that question for your reference –
Text from Previous post-

“Prime and Composite : Any integer which is divisible by 1 and itself only is called a prime number.
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N.B.: 1 is not a prime number.”

Question

Could you explaine what is the creteria thar excludes 1 from the list of prime numbers?
a) 1 is integer
b) 1 is divisible by 1 and itself (1)
Since anybody in the past has declared that 1 is not prime number, why we should follow this without thinking and contravene the general rule for prime numbers?
Is 1 as a figure is something which has come from the thin air. It is and always will be an integer. The criteria for 2 are the same – divisible by 1 and itself. And for all prime numbers.
Most probably the 1 is “guilty” because with 1 starts the series on numbers (natural, odd or prime). Suppose 2 was the beginning of the series. Should we ignore 2, because series starts with 2?