One theory that goes along with what's been said. They had terrible losses on the paperback Bibles. They just couldn't hold up to all that flipping to keep up at meetings and carrying them around in book bags. They had to make up for the loss. Introducing the "old" NWT back in the higher quality form would have just been met with Ho-Hums and not with money.
But to have a big event out of a new "silver sword." Letting everyone tie-in, oh the thrill of it all. They paid for these books and covered the loss from the paperback ones.
And, as long as they are at it, I am quite sure they changed some important words to say "Look at the new light we gathered from this."