Html is harder to learn then the others when it comes to people without any experience. It has tags and attributes, which can be hard to wrap your mind around. These Lightweight types are easier to use for a beginner. Wikipedia is not a wiki for developers, it is for users who have never made a website before.
Think about your audience, if it’s developers, they would be able to use html and have no problems with that. Although, they might then be able to interfere with your site code, which can be quite damaging. Leaving an a tag open, table open, javascript(!) etc.
Though, I have to say that it is a lot easer to do simple styling like bold and italic in bb code than html (specially if you are to make it xhtml strict valid)