It is rather common with zooms that the maximum aperture changes (in fact, decreases) with increasing focal lenght. At the same time, it is possible to make zooms with constant aperture (typically and not surprisingly, much more expensive).
What is the technical reason for the decreased aperture? After all it seems naively that the blades could be opened independently of what is happening to the focal length.
Is it to mantain the same level of image quality (distortion, aberrations, vignetting) at each focal length?
Or is there some "more fundamental" reason for this engineering decision, which can be overcome only in more complex/expensive designs?