Because the only people that use Markdown are programmers. I know, we think we're the center of the universe but the other 99.999% of the population have no ide

Because the only people that use Markdown are programmers. I know, we think we're the center of the universe but the other 99.999% of the population have no idea what it is, nor do most care to learn it. And Apple designs for masses, not niches.