Why would some one spend $1000 on a 70-200 f/2.8L? It doesn't have as great a zoom range as those two, but it has a constant aperture of f/2.8 and the Image quality is fantastic.
There are a lot of differences, from weight to sharpness and price.
It's like you as why buy a Ferrari when you can buy a Lamborghini when Lamborghini has more horsepower. Well there are a lot of facts to consider.
P.S. The example was for the demonstration purpose only, and i'm not experienced in cars, that is why i didn't mentioned any car models.