[thxbb12]

The reason is because the apparent aperture needs to stay constant when wide open. At 16mm it needs to be 16/4=4mm, at 70mm it needs to be 70/4=17.5mm. To achieve that one needs to use a few tricks which give one less freedom in design. 

 

So what you're saying is that a 16-70 f2.8-4 is easier to design than a 16-70 f4?

Following your example above: 16/2.8 = 5.7mm, 70/4 = 17.5mm. In which way does it make it more difficult than 4mm and 17.5mm?

I don't see who would actually rather have a constant f4 lens over a f2.8-4 one. That seems very silly IMO.