Why didn't Melissa Gilbert reprise her role as Batgirl?

She also didn't comeback for the "SubZero" movie.

https://www.cbr.com/batgirl-robin-starred-in-final-batman-animated-series-episode/

Not only does the episode feature Melissa Gilbert's last performance as Batgirl, but it was nearly the final performance of everyone in their roles -- "Batgirl Returns" was the final episode of the show's original run. FOX had completed its extended order for the series, and Warner Brothers didn't see a compelling reason to make more episodes. Focus soon shifted to a Superman animated series, partially inspired by the belief that a live-action Superman film would be released by the mid-90s. "Batman: The Animated Series" closed down production for two years, and wasn't revived until the WB network made a decision to pair new "Batman" and "Superman" episodes in a block of programming.