Forbes wanted to focus on movies, and knew that being a regular on a series like DS9 would tie her down too much for film roles. And her film career was in pretty good place at the time, as she had major roles in 1993's Kalifornia with Brad Pitt, Juliette Lewis, and David Duchovny, and 1994's Swimming with Sharks, with Kevin Spacey and Frank Whaley. It was not unrealistic to imagine she might be going on to bigger and better things.
She'd obviously changed her mind about doing series TV just a few years later though, as she signed on as a series regular on NBC's Homicide: Life on the Street as Dr. Julianna Cox in 1996. Perhaps her film career had cooled off enough that getting a steady paycheck seemed like a more appealing option than it did in 1992-93. Or maybe she just liked the material better, as H:LOTS was one of the most critically-acclaimed shows on TV at the time. She left the show after only two years and 30 episodes, though. I'd always assumed that it was her own choice, but Wikipedia says that she was let go.