It's because we've disabled the use of HTML for security reasons.
BBCodes are essentially almost the same as HTML, but they only work on posts and not the whole page.
IE:
CODE
<b>bold text</b> = [b]bold text[/b]
Bold text won't do much to security, but let's add javascript or soemthing like that and we have major security issues.
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And then the ever present threat of someone maknig a heat seeking bold missile script.