I can't think of a rational reason why the AG challenged the travel ban.  One of the reasons given initially was that it would hurt tourism from those countries on the ban list. I don't have the numbers but the numbers of tourists from these nations are probably statistically insignificant.  The real reason was so AG Chin could demonstrate to all his progressive buddies that he was doing his part as a member of the "Trump Resistance".  I could be wrong but I think it may have backfired on him.