It continues to sell well because it's a very good lens, and the only 'affordable' way to get 400mm with IS.
+1 I wouldn't buy the 400mm f/5.6 prime because of 2 reasons
a) it doesn't have IS and
b) isn't a zoom
Most of my telephone zoom photos - eg birds - are often up close, or where zooming helps in composing / capturing the subject (eg BIF - then zooming in)
So I can understand how for many the 100-400mm L meets their needs.
For me, I much prefer the design (ie size, shape, zoom mechanism) of the 70-300mm L. The 'big deal' is that I can transport my 70-300mm L in my Lowepro shoulder bag, with my 7D and 15-85mm lens (either lens attached) and shoot all day with it. This is my perfect 'travel zoom combination' - and I might throw in a prime (or my UWA) for certain situations.
More often than not however, my 2 lens combo is the 70-300mm L and 15-85mm. The 100-400mm L is substantially longer and more difficult to transport & carry than the 70-300mm L. Still I can understand it's a great lens for many ppl who really want that 400mm reach. The 100-400mm L isn't quite as sharp as the 70-300mm L nor has the newer IS or quite the same AF speed / accuracy (just a bit slower / more hunting in my experience)- but the 100-400mm L is still a very decent lens!