Why is the plot more important than characters?

It depends on what kind of story you're trying to write. For fast pace novels, the plot is slightly more important, although you can't afford to neglect the characters. For novels with a slower pace, you can focus more on characterisation, although again you can't neglect the plot. Generally it depends on what kind of a writer you are; I tend to give plot the slight edge over character while keeping the characters strong, but one person I know will sacrifice plot for character, another finds setting and atmosphere the most important, and I used to mostly disregard character.