because right now everybody is thinking about domestic issues and the economy. Switching as they did only plays into McCain's hand and no satisfactory explanation has been given to why they switched topics. Surveys show that the first debate is always the highest rated.
here's a story on the switch:
Whether it was John McCain's campaign pushing to switch the topic of the first presidential debate to foreign policy and national security or Barack Obama's campaign relishing the chance to go after McCain's perceived strength, the University of Mississippi's preparations for the Sept. 26 event received a curveball on Thursday.
For months, the Commission on Presidential Debates had assured Ole Miss that the focus of the debate at its Gertrude C. Ford Center would be on domestic issues. However, a joint announcement Thursday from the Republican and Democratic presidential campaigns said negotiations on the debates had resulted in a switch.
http://www.commercialappeal.com/news/2008/aug/21/21debateWEB/