It's due in part to the design. The focal length being shorter than the distance from the back of that lens to the sensor causes the need to add extra elements for it to work. It requires much more advanced design inside than a lot of the tele zooms. With wide angle lenses, not only do you have less space, but also you need more glass to bend the light further, squeezing up to 114° of view into a 24x36 area. So it's just plain harder to make a fast wide angle lens.
When you bend light rays you get chromatic aberrations.... basically, the light wants to split like a rainbow. You add multicoating, use expensive glass, and add elements (other pieces of glass) to reform the light rays back into its proper form. Because wide angles lenses do more bending, you need to reform the light rays more aggressively. Because the front element is typically larger, you need more light reforming material.
That's the general gist of the problem. As Optics design and mathematics is quite complex. And designing around the inherit limitations of optics and glass has kept scientist busy for centuries.
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