Because I think, that there are other formats, whose formatting rules are more intuitive than Markdown's formatting rules. For example:

    (1) I can never remember, whether I need to use [] or () first, when embedding an image. In Org mode syntax one simply uses [] for both parts.
    (2) Or whether ** is bold or italic or * is bold or italic. Other formats use the well known / to mean italic, which optically already makes more sense and also doesn't interfere with something being bold or not, because it is a separate symbol. This makes Markdown also harder to parse btw..