Because Adobe decided upon a non-standard preference dialog, both in where it's located and how you get to it. At the bottom of the ACR dialog that comes up when you select a NEF to convert, you'll see a blue Web hyperlink-like set of text at the bottom middle of the dialog.
That text contains a bunch of specifications about how the NEF will be converted. I'll bet it says 8 bit. Click on the link and you get...no, not a Web site, but a hidden Preferences dialog. Make your changes there.
Note that these items in the dialog are Preferences, so they are sticky. Next time you open a NEF you'll get the same things you select here.
BTW, while we're here, everyone should turn on the Open as Smart Object option. Smart Objects (and Smart Filters and all the other Smartness that goes along with them, including Smart Filter Masks), are a massively deep subject and tough to get your mind around, just like Layers were when they first appeared. But they are amazing things that suddenly make changing your mind very easy.
