Nvidia gimps the OCability of the mid range cards so that they won't step on the toes of their over priced high end chips. The only down side is that the new mid range cards don't look very good compared to last gen's OC'd high end cards, but when you are trying to rip off your entire customer base twice each generation, something has to give a little and the 1070/1080's poor OCability is what had to give, same as with the 600/700 series.
I was told by a couple different folks I know from other forums that usually "dig deep" in to the bios of different nvidia cards at the hex level. And I was told that nvidia permanently limits the voltage of pretty much the entire 700 line up of cards to 1.212v And nothing can change that. Not software, not bios mods.
However there were a few notable examples of certain board partners that released versions that ignored (and violated) nvidia's mandate and allowed the voltage to be clocked much higher. One was the EVGA 780 Ti Classified, and EVGA 780 Ti K|NGP|N editions. And (I think) Galaxy HOF / Hall Of Fame editions. There may be a few more.. but typically you could count these "unlocked" cards on both hands.
I think I heard EVGA got in "hot water" with nvidia over all of this and isn't allowed to do this sort of thing any more. But I can't find proof of this anywhere.
Locking the voltage like that locks the overclock. And I remember hearing that the voltage changes with the 900 series were similarly locked and even if we could push voltage, it didn't go very far, like +0.050 mv for example.
So I'm not terribly surprised seeing nvidia do this with their mid-tier line of cards this time around.
Part of it is planned obsolescence. It forces people to buy a higher tier of card if they want the higher performance.. instead of buying a lower tier card and overclocking it to the sky and getting the same performance as the higher cards for less money.
I do believe those days for nvidia are long over now and this will be the "new norm".
I may be wrong on all of these assumptions as they're just rumors I hear between friends but it is appearing to be that the 1080's are all voltage capped in some form that isn't changeable without "hard mods" (soldering things to the cards), at least so far. Almost all GTX 1080's overclock to the 2ghz - 2.1 ghz range even founders edition and the only difference in aftermarket ones is better cooling. Even additional power connectors don't let us clock further past a certain point.