All the half-dragons in 3.5 would point to it having little to do with cross-fertility. In a magical world it could be just because - it's their nature, essence can come before existence in fantasy logic, even for thinking beings. Since they're sentient it could be a taboo. Since they're arrogant, it could be feeling other-colored dragons are inferior (or in the case of weaker dragon types, being unable to bear being the weaker partner in a 'mixed' mating). There could be an arcane reason - maybe the juxtaposition of the elemental forces that power different breath weapons would make it unpleasant or dangerous. Or, more mundanely, as with some technically-separate species IRL, it could just be because their ranges are different, Red dragons never visit the tundra, White dragons never visit volcanoes.
Or, maybe they do, but the result is always one color or the other, not a mix.