Range and time over station (TOS).
Predators don't hold a lot of gas. Hanging a couple of weapons significantly decreases range.
I'm not say that why this sortie was without weapons...just offering one option.
U.S. engagement in Libya consisted mostly of aerial missions. More than 70 U.S. aircraft were involved in the Libya operation -- throughout, the U.S. conducted a total of 145 Predator strikes in the country, in addition to thousands of other aerial missions. The United States has stepped up the use of Predator drones in several other countries, including Pakistan and Yemen. A drone was credited with killing American-born radical cleric Anwar al-Awlaki in September.