It's probably because the marketing people believe the consumer prefers red grapefruit. Once again variety is sacrificed by mass distribution and marketing.
Besides, the Oro Blanco is the new white grapefruit now (even though it's more of specialty variety and is seasonal).
Besides, there are so many options now, they can't carry them all, and sometimes varieties are displaced by newer varieties. In this age of mass distribution and commercialism, there's a push for everything to be uniform. So it's not surprising there is only one kind of grapefruit sold in supermarkets across the country. People only know what a "grapefruit" is, they don't have time to think about different varieties. It's really because there's no one who cares, and the decision-markers in charge of grapefruits are far removed from the consumer.
