Q. Did the customers stop for narrow selection of food?
A. The customers stopped for wider selection of food.
Does it mean the customers just ignored or didn't shop for food?
To "stop for (something)" means to make a stop in your travels in order to do, buy, or accommodate something. You could say, for instance, "We stopped for a drink on the way home" or "while we were out walking around the block, we stopped for a chat with a neighbor."
So in the context given, "stop for" means the customers specifically came into the store, rather than choosing to pass it by, because of the store's wider selection of food: They made a stop in their travels in order to buy food there.
In the given sentence and without further context I'd imagine it means that the customers came (to a particular shop or salesman) for the wider selection of food that is available at the particular shop. So the sentences should be:
Q. Why did more customers come?
A. The customers came for the wider selection of food
A. The customers came because of the wider selection of food