Core77

Answer: Patriotism. As you can see in the photo above, the logo was initially all red. That changed inl the 1940s with the advent of World War II:

To show support of the war, Pepsi unveiled a new bottle cap that featured the Pepsi script surrounded by swirling red and blue colors on a white background. Since Pepsi was recognizable with its script logo in the same manner as its main rival Coca-Cola at the time, the cap logo was simply meant as a show of U.S. patriotism as opposed to a marketing scheme.

The cap logo, however, quickly caught on, and by the end of the war in 1945 became Pepsi's primary logo....

via uk logo design