After sleeping all night for 6-8 hours, or whatever amount is required for you personally, the body, upon arising in the morning, is in a very catabolic state. This means a state of negative nitrogen balance, something no bodybuilder wants.
To perform cardio at this time, on an empty stomach, creates a further state of catabolism, thus resulting in muscle breakdown, and muscle tissue loss. The exact opposite of what bodybuilding is all about! It's true that blood sugar levels are low in the morning, but so are amino acids, the building blocks of protein.