There's nothing more to it than that? What I'm looking for are reasons more like that it generally makes common operations simpler, less off-by-one adjustments, etc.
For comparison, in the positive array index case, experience has shown that it's generally more convenient to start from zero, and to use python style ranges where the bounds are specified as low <= index < high, i.e. the high bound is one off the end of the range.
So I thought there might be similar experience in the from-the-end indexing case that is behind the choice to start at 1.