Could be because there isn't one in that colour. A cherry pink one is a recent introduction so it is going to be the nearest to red you can get.

I dont think there is a pure red one yet so that will be why as amblealice says.

They are also known as Calibrachoa. If you are buying form garden centres/nurseries, its best to go by the flower colour as the labels can be completely different than the true colour! I know this form own experience at the nursery, order 'Bubble Gum' with a pink label and picture in catalogue and they turn out to be pinky/purple.