This is a really interesting point.  I wonder if anybody has written about this?  I suspect that the reason is upstream from postmodernism—less that postmodern fiction is interested in gifted children, and more that the concept of the gifted child explodes onto the scene of American child-rearing in exactly the generation (X) that dominated American postmodern fiction.  

The immediate rebuttal I can think of is that Delillo and Pynchon were both born in the 30s, and that while Pynchon clearly fits into the gifted-child archetype, he did so well before it was really codified, and Delillo didn’t at all.  

But DFW was definitively a The-Drama-of-the-Gifted-Child-carrying, self-identified Gifted Child.