The reason for this is that such manufacturers believed that this would be the best way to sell their products.  

During the war, everyone wanted to look as if they were patriotic and as if they were doing their part.  Many manufacturers tried to play up their connection, if any, to the armed forces.  By doing so, they hoped to build up a good reputation among consumers so that consumers would be more likely to buy their products.  This was especially true for firms like Chrysler which essentially could produce cars during the war because its factories were needed for war production and so were the raw materials that could be made into cars.