Good point Stuart.
Since Bennu is located between 0.9 AU and 1.4 AU from the Sun, it is in a relatively harsh environment exposed to the solar wind and measuring only about 500m across it has an extremely low gravity. So since it is likely to be billions of years old, there is a lot of time for any dust between the loose assemblage of boulders to be lost. As to the mechanism for this, electrostatic charging is a good one, as you suggest.
One thing to note when looking at the boulders is that they are not like terrestrial rocks since they are very black (4.6% albedo) but more especially the bulk density of Bennu is only 1.26+/-0.07 g/cc. So as there are a lot of voids between the boulders, their density will be roughly 1.8 g/cc on average. This value is similar to the lightest meteorites (certain carbonaceous chondrites have a density of about 2.1 g/cc). My guess is that many of the boulders comprise some sort of porous carbon material mixed with silicaceous grains.
Next year, a sample is scheduled to be taken and the scientists are looking for areas on its exterior where fine-grain material is present and where they can avoid boulders. They have found 4 possibilities:
https://www.space.com/asteroid-bennu-landing-site-options-nasa-osiris-rex.html