The missing voters were Democrats. The meme in 2012 was that *conservatives* stayed home, but Romney got a million more votes than McCain from a smaller electorate.
I suggest you read Dick Morris's book Armegeddon. He disagrees with your take. He says that the white voter turnout was disappointingly low while black and other minority voters came out in record numbers to support Zero. IIRC, Morris said that about 10 million whites stayed home, and Trump's opportunity would be to get these whites out in 2016. (The book was published this past June.) Morris was pretty close to what eventually happened.