I would suspect that cost is a major factor. You have to look at WHO is buying the new homes these days. They may be Techo-smarties and the like but at the same time, they're looking at the cost of wiring a home.
You see, I think it would be better for a builder to, instead of doing the wiring, 'rig' the house for that potential. In other words, how 'bout installing junction boxes in certain locations along with tubing? How 'bout running a couple of 1" tubing from the basement (where the electrical panel is) up to the attic so you can easily 'fish' in the wiring at a later date? In doing this, you don't add an at-the-time unnecessary cost of wiring the home, but rather you install the POTENTIAL for doing such. It's hard to convince many folks (educated ones too) that installing something like this is worth it if they AREN'T going to use it. Yes, I agree w/you that it's nice to have, but you can't convince me it's worth the $$$ (now) to spend if I'm NOT going to get any use out of it. Look at flat screen TVs, and Digital Cable, and so on. These 'things' are being slowly introduced into our society but haven't taken off yet. So, until that happens, I don't think you'll see Contractors adding costs to their home when their buyers aren't asking for it.
Instead of trying to convince a Contractor that this is the way to go, how 'bout educating the general public instead? They are the ones that ultimately pay the $$$. (I guess I'm saying that many Builders know about HDS but, again, they aren't the ones needing convincing.)