This phone has a 1080p screen, torrenting a 1080p full Hd movie will take at least 4GB with some blueray rip around 7GB. In Droid DNA, you have 16GB with usable storage of around 11GB, minus all the other stuff on the phone such as photo, books etc you can only fit one full HD movie to take advantage of the 1080p screen.
When you put it in that context, the conspiracy theorist begins to wonder if there is a MPAA influence being felt here?
The truth of the matter though is that Google and more specifically manufactuers are not really concerned about the very small (the word infintisimal comes to mind) percentage of people who want to carry 64+ GB of 1080p movies on their device. They are concerned about the experience of the average user though who doesn't even use the 16GB built into the device. If their research shows SD cards are harmful to that experience and it saves money not to include it - out it goes.
Fortunately, Android still supports it so there will probably always be some manufacturer making a device for the people who want an SD card.