There was a POV of Elayne earlier that explains why she wouldn't do that. She thinks somethign along the line of "If the Queen starts breaking her own laws, justice itself dies". For Elayne it's a pretty fundamental principle.

Sylvase did try to tempt Elayne into using Lounalt's services (it suggests Sylvase was indeed a DF trying to get access to the dungeons for Lounalt, unless Lounalt planted that in his head but unlike Sylvase he shouldn't have known about the prisoners at that point), Elayne brushed it off as a dangerous path. Some thought it was foreshadowing Elayne would give in (like some thought Nynave would go wild on Semirhage and break her, totally forgetting Rand's moral stance on torture. Nynaeve pretty much stood aside and let Cadsuane deal with it in the end), but Elayne of course is now Queen and didn't even return to that topic.

It's also a line RJ tended to want the Light's characters not to cross, and when they did the price could be heavy (both Adeleas and Vandene died, breaking Ronde Macura had unforeseen consequences etc.), usually the heavier the less restrained or digusted with themselves the Lightsiders who use torture were (and the "wronger" they smell to Perrin's nose). He even made it the last moral straws Rand held to, the last line he wouldn't cross (he made his moral stance on torture in Tear early on, making it illegal. He never wavered, not even against Semirhage. Toward the end he became really cruel and sacrificed people without blinking, but he didn't torture) and made it the moral breaking point for Perrin, who decided now he hated his axe after using it for torture. It's also the act that so horrified Aram he started listening to Masema.