Because Gruber used a different convention? > While Markdown’s syntax has been influenced by several existing text-to-HTML filters — including Setext, atx, Tex

Because Gruber used a different convention? > While Markdown’s syntax has been influenced by several existing text-to-HTML filters — including Setext, atx, Textile, reStructuredText, Grutatext, and EtText — the single biggest source of inspiration for Markdown’s syntax is the format of plain text email.