Butler might have had Kevin stay in the past to get a better understanding of Dana, and to give Dana more time in the past to fully experience the lives of her slave ancestors.
Kevin was understanding, but concerned, when they realized Dana was time traveling. He was also concerned when he figured out that she could only return when her life was threatened. Yet when Kevin is holding Dana and they are both transported back to the past, Kevin develops more of an understanding of her situation. He was always a loving and empathetic husband, but now he could fully understand.
Another reason for having Kevin stay is to keep Dana more connected to the past. He goes off and she is left behind. Rufus says he is sending her letters to Kevin, but he actually is not.
Kevin might have my letter by now. He almost surely did. He might already be on his way here. (p. 156)
Kevin’s absence, and Dana’s disappearing, do not seem to separate them. In fact, Dana says that she and Kevin have grown closer together, and their relationship is not damaged by the missing time.