 Why do Bob and Quezy haul asteroids in the first place?
And if The Interplanetary Hauling & Moving Co., hadn't been about to go on the rocks (chiefly due to the activities of Saylor & Saylor, a rival firm) neither Bob nor Queazy would have thought of sending an answering ethergram to Burnside stating that they would fill the order. And yet, if by some chance there was such a rigidly specified asteroid, their financial worries would be over. Now that he and Queazy had found the asteroid, they were desperate to get it to its destination, for fear that the Saylor brothers might get wind of what was going on, and try to beat them out of their profits. Now they scuffed along the smooth-plane topside of the asteroid, the magnets in their shoes keeping them from stepping off into space.
 It's a new fad that Bob hard started, where rich people enjoy having them on display.
 They are hoping to start a new business selling them.  Other companies have been making a profit with them, and they want in on it.  The asteroids of deposits of rich minerals, making them valuable. Hence why they check the composition of each one.  It's a new fad that Bob hard started, where rich people enjoy having them on display.