WHAT DO YOU MEAN THEIR HAIR IS AMAZING.
... I'm kidding, of course.
I dunno. Maybe Mars did what I do; cross her fingers and hoper her karma wasn't bad enough that Bronzong would hit itself. (If I decide to barrel up to the Elite Four without any training after the 8th Gym, I'm typically using the same strategy by the third trainer.)
Or maybe she'd loaned her last Full Restore to Jupiter. Or maybe she actually hates Bronzong as much as the rest of us do and gets a perverse pleasure out of watching hers faint. Or maybe on an unconscious level she wants to see Cyrus's plan fail. Or maybe she was trying to get a Full Restore out of her pocket but couldn't get it out before Bronzong hit itself. (Hey, that dress is pretty skin-tight.)
Hey, everyone else has contributed the answers that use "game mechanics" logic. (Everyone else has contributed the same answers about ten times. ¬.¬) I'll gladly provide the "in the game universe" logic.