Because the disgusting taste of grapefruit juice overwhelms every other taste.
Ever since childhood, I have not understood why people seem to like grapefruits amd grapefruit juice.
Different people taste things differently. There are several taste tests used in biology classes to show this to students. Here's one of many references: http://www.ns.purchase.edu/biology/bio1550lab/ptc_tasting.htm
Put some splenda on one and try it. They are yummy. This coming from a picky eater. It suprised me. I only tried it because of a diet. Now I eat them alot. Just bought some last night.