I always kind of figured (and I thought I'd heard it somewhere fairly reliable) that it was the chance to be on a sitcom with a big sitcom star.
She didn't get the gig on Becker until the DS9 negotiations were over.
However, I just did a bit more research regarding those negotiations. I didn't realize she had been paid less from the start of DS9 and was just trying to get equal pay. And, Paramount screwed her over when she was on Becker, too.
I don't blame her for more-or-less leaving acting after that experience.
Kindly disregard my previous rants. It'll take me time to readjust my attitude toward Jadzia/Farrell - but it seems I need to.