Here are a few reasons that spring to my mind:
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PHP/MySQL/Apache based, so will run of virtually any web server, VPS or shared hosting.
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Now fully open source and actively developed.
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Single sign on to integrate with an existing member database (e.g. a forum).
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You can change as much or as little as you like with CSS themes and advanced themes.
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Fine-grained control with advanced themes, so you can change a few bits of code or completely move things around.
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Ability to add custom pages (e.g. for FAQs/Rules)
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Good permissions control (e.g. who can ask/answer questions, edit and delete).
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Plugins! You can add different login/editor/page plugins, though there are not a huge number developed yet.
