Harold McGee describes a bit of tea history in On Food and Cooking...

when China began to resist Britain's practice of paying for its expensive tea habit with opium, the British intensified the production in their own colonies, particularly India. For warm regions, they cultivated an indigenous variety Camellia sinensis var. assamica, or Assam tea, which has more phenolic compounds and caffeine than China tea and produces a stronger, darker black tea

He goes on to describe other cultivations and processing methods, but the gist of the reason is that this type of tea became popular because it was relatively easy to grow and preserve. As an aside, even black teas shouldn't taste unpleasantly bitter if brewed properly and from minimally damaged leaves.