A Couple of guesses here:
The gear hanging out in the breeze helps generate drag (which can be a good thing in aerobatics...you might not want a plane that picks up speed in a hurry when you point the nose below the horizon). Also, a hydraulic pump (for a conventional single) would be an added, unecessary complication.
BTW, I can think of one common aerobatic aircraft with retracts (although it is ex-military): the T-34:
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Photo © C. Nolan Huizenga