Because after he and Gohan came out of the Hyperbolic Time Chamber, they'd achieved the art of staying Super Saiyan all the time - even when sleeping. It was almost like they COULDN'T go normal. This was part of the canon when they were preparing for the Cell Games.

Krillin found out to his detriment that they weren't hyper-aware of everything when sleeping though. The episode where he threw a rock at Goku's head, expecting him to blast it straight out of the air, was hilarious.

...shit, this was fucking six years ago that this series was my life and I STILL REMEMBER EVERYTHING. WTF?!