Because I already have a 70-200 2.8 IS that covers this range and I am not a big fan of extenders (although this could change with the new mark III's).
But the 200 f2 is a very different league than the 70-200's, much better and
Brighter. You have right that a 70-200 maybe not the most optimal solution is with extender.
And he is 1000 Euro cheaper than the 400do and 300II..