Because it is not how computers work. It doesn't matter much for high level languages like LUA, you rarely manipulate raw bytes and pointers, but in system programming languages like Zig, it matters.

To use the terminology from the article, with 0-based indexing, offset = index * node_size. If it was 1-based, you would have offset = (index - 1) * node_size + 1.

And it became a convention even for high level languages, because no matter what you prefer, inconsistency is even worse. An interesting case is Perl, which, in classic Perl fashion, lets you choose by setting the $[ variable. Most people, even Perl programmers consider it a terrible feature and 0-based indexing is used by default.