cinni wrote:I could be really wrong. But. I think it's because rooibos doesn't have any tanins. That's the stuff that makes 'real' tea, like black tea, bitter if you steep it too long.
Actually, tea has very few tannins-- it's a common misconception. The bitterness in tea comes from similar compounds called polyphenols. IIRC, tannins are polyphenols but not all polyphenols are tannins. There is also some debate about the nomenclature "tannin", but that is a different issue. I would guess that the word "tannic" was adopted from the wine-tasting lexicon because of a similar kind of bitterness found in the two beverages, but in reality the tannins that are present in wine are essentially absent in tea.
That said, I think you are on the right track. Tea probably has some bitter compounds (presumably polyphenols) that only dissolve in large quantities after the tea has infused past a certain equilibrium point. I would guess that rooibos (and many other herbs) do not have much of these bitter compounds, or their bitter compounds are not soluble in water.