While I don't speak for anyone at SIAI, several possibilities spring to mind.

• Accepting bitcoins relatively early in the game would tend to raise the status of an organization like SIAI should it prove popular, but doesn't cost it much if it doesn't.
• If the value goes up the organization can profit financially by holding onto them.
• It may be the case that people will donate bitcoins more easily than equal value in dollars, for psychological reasons (e.g. lack of equivalent ugh fields).
• It may be that people who initially consider donating in bitcoins will change their minds and donate dollars instead.
• It could plausibly come about that processing power, and/or the time and energy of AI hackers, can be purchased more cheaply/easily for bitcoin than dollars.