The answer, really, is pretty logical. A pressurized transformer would be more likely to force oil out of the tank if a leak developed.
Oil spills are unhappy events.
They are messy, and sometimes cause environmental problems. Positive pressure, then, may have a disadvantage. Drawing an oil sample is easier, though, when the transformer tank is pressurized, and a leak would not result in contaminants being drawn into the transformer. Let's think about negative pressure. If that air were laden with moisture, dust or other impurities, it could contaminate the oil, if maybe just a little bit each time.