As is the case with Markdown, many parsers have prioritized ease of implementation over formal rigor.
Italic headings? Bold links? Nested lists - how many levels? Code in list? How do paragraphs interact with lists? There are many opinions and many leaky implementations of those opinions. Newlines? Embedding HTML in Markdown !?!?
It all seems so sad, because (X)HTML nailed most of these issues a very long time ago. But HTML implementations were sloppy from the outset. And XML was born with inherited bloat, then got ever more complex over time (modular specs, XLink, XPath, XSLT, DTD -> XML Schema, ...)
With Markdown, it is relatively easy to introduce some recursion into the parser, but for what spec? In what contextual cases? At what cost?
It is possible to just treat commas as whitespace. It makes implementation so much easier. It accepts missing, trailing and repeated commas. It makes elements uniform. It ignores many common errors that arise from typos or cut'n'paste. It makes JSON writers simpler, by removing the first/last special case.
A JSON parser that treats commas as whitespace can be two dozen lines in most programming languages - if you do not want line/column, chapter and verse, for the remaining error messages.