I don't know and sadly have never flown a Pitts either. I did try to look it up on the Society of Experimental Test Pilots papers database, but surprisingly there are no papers there on any of the Pitts designs.
I'll offer a best guess however, just from having run a lot of certification and flight test programmes. Probably, the flight test team didn't see enough need to fly aeros with anything in the wing tanks, so couldn't justify the time and resources to test that. Since it wasn't tested, it's prohibited. As I said, guesswork, but it would be consistent with a lot of other restrictions I've known of in other aeroplanes.
G