Two theories try to explain this phenomenon: "passive queuing" and "active monitoring." The former says that information has to line up for a chance at being processed at some focal point of the brain, while the latter suggests that the brain can process two things at once it just needs to use a complicated mechanism to keep the two processes separate. Recent research from MIT points to the former as an explanation.
Yuhong Jiang, Rebecca Saxe and Nancy Kanwisher, in a study to be published in the June issue of Psychological Science, a journal of the American Psychological Society, examined the brain activity involved in multitasking. They gave people two simple tasks. Task one was identifying shapes, and for some subjects, task two was identifying letters, for others it was identifying colors. The subjects were forced to switch from one task to the other in either one and a half seconds or one tenth of a second. When they had to switch faster, subjects would take as much as twice as long to respond than when switching more slowly.
Using MRI technology, Jiang, Saxe and Kanwisher examined subjects' brain activity while performing these tasks. They observed no increase in the sort of activity that would be involved in keeping two thought processes separate when subjects had to switch faster. This suggests that there are no complicated mechanisms that allow people to perform two tasks at once. Instead, we have to perform the next task only after the last one is finished.