Quote Originally Posted by Anonymouswizard View Post
It depends on where you begin your index,...
The index was begun in 1974, with original D&D. It had wizard spells level 1-6, and cleric spells level 1-5.

Then, starting with Greyhawk in 1975, higher level spells were added, and they always continued the numbering system. Various spells moved around, but the base system was in place in 1974.

A lower level below level 1 was added eleven years later, in Unearthed Arcana. That's why they are called 0th level.

You have some good logical ideas about what other numbering systems could have been used if the entire modern system were invented from scratch today. But the actual decision is was based on the previously established system.