And then there's Google's explanation.
Why don’t Nexus devices have SD cards? Everybody likes the idea of having an SD card, but in reality it’s just confusing for users. If you’re saving photos, videos or music, where does it go? Is it on your phone? Or on your card? Should there be a setting? Prompt everytime? What happens to the experience when you swap out the card? It’s just too complicated. We take a different approach. Your Nexus has a fixed amount of space and your apps just seamlessly use it for you without you ever having to worry about files or volumes or any of that techy nonsense left over from the paleolithic era of computing. With a Nexus you know exactly how much storage you get upfront and you can decide what’s the right size for you. That’s simple and good for users.