How is it not powerful enough? The V8 supercar engine as far as I know isn't nearly as restricted in power as the LMP2 VK, they're both on the same architecture of engine design, and the LMP2 version and full tilt is capable of 800hp, so it stands to reason that "isn't powerful enough", isn't all that correct.
Also if it's a TTV6 then the 3.0 from the LMP1 makes sense, but if that was the case then Nissan could have just stuck to the previous race engine TTV6, and yet didn't because they liked the benefits of the N/A V8. So I'm finding it hard to see why they'd go back on the VK45 for a new V6 twin turbo, when they already have two of those for racing spec. And the one that they may use is only 40-50 more in horsepower. I don't see how the VK56 can be used as reasons stated above, unless they 5.0 rule still exists for max capacity of engines going into the DPi era.
It seems logical as
@RACECAR put it to keep using the VK45 since the engine is more than powerful enough if Nissan wanted it to be. Clearly where it stands now is more advantageous race wise, and probably why so many use it.