I don't pretend to understand the feelings of people who care about this issue strongly, so I won't defend the choice vigorously. But I would guess they did that because their target demographic were Fortran and Matlab users, all of whom use 1-based indices, as is usual in math books. At any rate, in Julia you rarely need to write the start index explicitly (because of functions like eachindex) and can easily start your indices anywhere you like if you feel strongly about it.

(I know that to you specifically the fact that the ability to choose the starting index is not builtin means it doesn't matter, but I point it out for others who might not know that it's easy to do in Julia.)