There's some sort of threshold which is reached at 400mm and f/5.6. There are several ways of getting there for ~£1000; for example in the Canon range alone:
* 400mm f/5.6 L
* 100-400mm f/4.5-5.6 L IS
* 300mm f/4 L IS + 1.4x TC
* 200mm f/2.8 L + 2x TC
* 70-200mm f/2.8 L (IS) + 2x TC
But if you want to go longer, or if you want to get to 400mm faster than f/5.6, it's going to cost you big time.
I wonder whether it's because all the above use 77mm front elements, which are relatively common, but 400mm at f/4 obviously requires a 100mm or larger front element: harder to make, therefore more expensive, therefore less popular, therefore made in smaller numbers, therefore even more expensives, therefore even less popular ....