Because Sprint/Google said so...
I'm speculating, but Sprint probably made the choice to only sell the16 GB version to keep down costs. And Google probably isn't making a ton of the red N5's so it's not worth it to ship them out to carriers. But it would probably be a better ? for Sprint or Google.
But luckily your problem can be solved by Google Play!
Nexus 5 4.4.2 (rooted)