Why would a man's coworkers make sexual comments about his wife if she is not flirty and does not dress provocatively?

Well, to be blunt, because they are immature, inconsiderate pigs. Sounds like maybe they are trying to get to him. The man needs to tell them that it is insulting to him for them to continue this behavior and they need to stop. May be they think it is a compliment, some men are small minded that way.

It is interesting that you point out that the woman did nothing to cause the behavior. It hints at old times when women were blamed for the bad behavior of men. In this modern world the man should complain to his supervisor about the illegal sexual harassment occurring in his workplace. If it continues he should complain to a higher authority in writing if necessary. Some men, like his co-workers, still return to their caves at night and they need stronger lessons about the rules in the more modern world.