This is a great question. Moreover, this is a very important topic for today's world. There are many points that the management could make not to want to cooperate with a unions.
First, the management might simply say that they have always treated people fairly and have given good benefits. They might say that it is not wise to tamper with something that works. Why mess with the status quo, especially when it works?
Second, they might say that unions are too powerful and that the workers will have an unfair advantage. They might bring up the fact that unions might actually make the company worse and hurt the company in the long run, which in turn would hurt the people who work there.
Third, they might look to other companies and say that these other companies do not have unions and so they do not need one as well.