I don't agree that the question changes. I would argue that the fact that Fortran enshrined the convention is a more relevant answer than the answer to why Fortran picked that range. The reason being that the answer to the latter question is most likely from another domain, such as Mathematics.

Although I do find the answer to the latter question interesting as well, just not as interesting as the answer to the first.

Because mathematicians, physicists, and engineers use i through n for integers in their mathematical formulas, and Fortran was their FORmula TRANslator into computerese. (Additional fun fact: They begin their matrix row and column counts with 1, which is why Fortran array indices start at 1.)

You didn't begin in column 7 but wisely avoided the continuation character.

TODAYISEVERYBODYTALKLIKEFORTRANDAY

FORTRAN taught me that languages should understand some spacing.

2H:)

Not only prefixed, I barely remember my grandfather telling me a story about a thousand years ago identifiers where so precious, they were only ONE char long, from A to H were asigned to strings, from I to P were for integers and the rest for floats.

Oh those long gone years...