I think a better question would be why she wouldn't outsell them? I mean, they're not pop queens. They're not massive in the music scene, their sales aren't unreachable.
Also, she's outselling them by... what, 10-15k? (if predictions are correct). She's doing really, really well, but she's not slaying.
They've all had similar starting positions, with similiar backgrounds and the same type of fanbase and they are selling in the same range. None of them are flopping, but none of them are absolutely slaying the competition either.
I think Ariana coming out on top might be because she has a more unique sound than what you generally hear on radio today (yes, Mariah Carey blah blah blah, but how many current artists have this particular sound?) while Selena and Demi have a more common sound. Not putting any value into it; just an observation.
They've all done well for themselves
Although I think Yours Truly is definitely the album that deserves the most success, so Ariana bettah snatch some weave with her longevity