Extremely interesting backstory from when Standard Markdown was announced: https://news.ycombinator.com/item?id=8264733

Basically a small group of people worked in private for years while there were public community efforts, including a W3C gorup, to do the same. None of the disparate efforts coalesced into a unified front, so the simple answer is that projects advocated for whatever spec their developers backed. The community is fractured.