She didn't for the same reason that she didn't trade weapons technology or prey on weaker species when it would have been advantageous to do so. It's a recurring theme throughout the series that Alpha quadrant ideals and ethics get in the way of Delta quadrant survival and the need to get home, and whether she as a captain, with a responsibility to ensure the well-being of her crew as well to uphold the values and moral standards of the Federation, has the right to choose one over the other, and what the right balance might be.
Even assuming that B'Elanna is as skilled an engineer as Scotty and Tuvok is as skilled a science officer (though he wasn't one) as Spock, violating the Temporal Prime Directive is magnitudes more dangerous than violating the Prime Directive.
Also, though the original Enterprise seemed capable of traveling back in time at will, the Enterprise-E, the Defiant, etc. seemed to rely on temporal anomalies and time portals created by others. So either Kirk's crew was very exceptional in being able to pull off the Slingshot effect, or the Federation realized time incursions were so risky and reckless that it was completely forbidden.
Also, the Barzan wormhole was never fully stable. Sure, one end of it was always at Barzan II, but the other end moved around randomly. It was hard enough just to surface the other end of the wormhole long enough send the 2 Ferengis back before it collapsed.
Plus, the only verified position of the other end of the Barzan wormhole was in the Gamma quadrant. If Janeway wanted to travel to the Gamma quadrant to get back, then she might as well use the Bajoran wormhole. And even if she had known about the past occurrence of the Barzan wormhole in the Delta quadrant near Takar (which she clearly didn't as the encounter with the Ferengis was purely by chance), it's possible that the sensitive wormhole could have been affected by the Slingshot effect, permanently destabilizing it once again. Then they'd end up both back in time and no closer to the Alpha quadrant.
And even if she had managed to go through the wormhole back in time, she would have undoubtedly bumped into the Barzans and the Federation/Caldonians/Chrysalians/Ferengi, thus irreparably contaminating the timeline.
Her safest bet would have been to go back in time and send a message to Starfleet to avoid being pulled into the Delta quadrant in the first place. But, again, Janeway was clearly unwilling to alter history like that, especially as she'd lose people like Chakotay, Seven of Nine, Kess, Neelix, and Naomi.