Why did the series get overhauled without Ben?

If I recall, it was mainly because the Linda Phillips character became the breakout character of the show. Therefore, when Duet's ratings sagged, I guess Fox just decided to cut their losses and make a new show around Linda. Ben and Jane became expendable in the Open House version. Laura was still meaningful enough to the audience to keep around and by that time she and Ben had separated. Shame really, I loved the show.