It's more a logistics problem from the manufacturer's standpoint. A fixed mag rifle required only one part to be modified/removed/installed - the magazine lock. On a featureless it requires much more work - a modified grip or wrap, a pinned stock, an alternate muzzle device, etc. So it was more investment in resources and time for the same end result.
Now, with the new laws going into effect, I'd imagine we will see a big shift towards featureless but the main issue is there are not a lot of elegant solutions and the better solutions are smaller companies that would have trouble supplying to the likes of River, S&W, etc. It seems to me like I see probably 2:1 or even 3:1 featureless to mag lock rifles when I go into stores, but that may just be my perception.
Honestly, I think the best thing that could happen to us is someone with a patented design to license it to a major OEM but I don't think that will happen.