One factor might be that during the time when gun control was actually relevant as a political issue the region had a strong presence of urban and suburban Republicans. Columbus, OH used to be a Republican city (its county voted Republican for President as recently as 1992) and Cincinnati is obvious. Indianapolis also used to be far more Republican than most cities (and Lugar is a former mayor of it.) Meanwhile this area at the time also had many rural Democrats.
That.
Gun control isn't a left/right issue.
It is a urban/rural one, mostly.
Look Northern Ontario (okay, it is in Canada, but the dynamics are similars). It is very rural, they vote for left-wing parties, but are opposed to gun control.
Look Toronto suburbs. Some are right-wing, but they support gun control.
It is an urban/rural divide.
It is logic, since in cities, hunters are rare, but gangs are present.
In rural areas, gangs are less of a problem and there is much more hunters.