Large corporations often feel they are too big to be effectively managed by a proactive security strategy. Since proactive security takes control of the whole organization, there are many areas that it cannot cover. For instance, all employees might not be trained about a certain attack before it happens.
In addition, large companies are usually concerned about their reputation. Proactive security adds extra eyes and ears that might eventually expose threats that face the organization. Moreover, some measures, such as an urgent request for all employees to undertake a certain training, might go out in a negative light to show that the company is under attack.