Why only by theft is someone who can’t pay back sold as a slave, and not by borrowing or damage or anything else? I know that’s what the Pasuk says, but is there a Meforush Pasuk that explains it?

See Shmot [=Exodus] 22 2: "He shall make full restitution; If he have nothing, he shall be sold FOR HIS THEFT". Since the passage deals with the obligations incurred by a thief, the words "for his theft" teach us that a man is sold only to repay the value of what he stole but he is not sold to repay any other obligation.