Probably a lot of flags contain red because red represents courage, revolution, hardiness, blood, and/or valor.
Red color also depicts sacrifice the hardships faced for a nation to be made. I believe there is a scientific reason too for having red among the colors in national flag. That is red being the color with the greatest wavelength in our visible spectrum red can be viewed from great distance and in case of the national flag which identifies a nation the color red makes sure that the flag is noticed.
