Stephanie (both in real life and as an on-screen persona) considers herself several pedestals higher than the rest of the WWE Divas, so I can't imagine that she'd want to compete for the Diva's Championship like the rest of the herd. Can you even picture her carrying that horrible-looking butterfly title around on her shoulders? If we're being technical, Stephanie doesn't really consider herself a 'Diva' anyway, so competing for a title that has the word 'Diva' in it would be 'beneath' her.
Furthermore, she'll be on the wrong side of 40 in just over a year from now, so I doubt she wants to step into the ring on a frequent basis like being the Diva's Champion would require her to do. Even when she was the Women's Champion for a year when she was around the age of 23 or 24, she only defended it one time and that was the night that she lost it lol.
The closet permutation to this idea I can see is Stephanie putting over whoever the Diva's Champion is at a given time in a grudge match or something (Stephanie putting over one of the other women in a match? Bwahahaha... Like that would ever happen.)