Ive seen lately that this les is one of canons most undersold lenses, why.
The only reason I've heard why people haven't upgraded, is because most people with a 70-300 or 75-300 go to a 70-200 f4. But this lens is even cheaper, and it is 2.8.
You say why, I'll chime in on similar note as many others. The smaller 200 is not a zoom, it's not white (aka. not cool), and probably not many user of the lens you mentioned realise the benefit of AF speed gained by upgrading to f/2.8.
I own this lens for almost 2 years, and this is "my precious" (for its price and its quality). I had the dilemma whether to take this or 70-200/4. Both has no IS, but I jumped on the smaller 200 because:
it's black (inconspicuous; still a head-turner with its hood on, though)
it's lighter (must check the numbers though, but on 20D, its light enough in my feeble hand),
Virtually against most people's logic behind the move to 70-200. But why go mainstream?