orkut Blog: Redesign Preview: True Blue Simplicity
colour that reflected to you mind when you see Google : primary colors, i.e, red, blue and green.
colour that reflected to your mind when you see orkut: blue
this may be one of the reason Google kept its default theme colour to blue.
Though now you can change the theme of orkut pages depending on your mood, on the other side blue colour reflects orkut mood, its everlasting simplicity and user friendly nature.
As the time is changing, orkut is also changing but the only thing that will remain the same is user friendly nature of orkut.
So still Google kept Orkut background colour to blue with whitish touch.