From her interview with Radio Times, Charlotte Ritchie said she wanted to leave but would have ideally liked the 'door left open'... as they have for most of the other actors who have left. BUT... what would happen to Babs & Tom?They've only just married, so they couldn't split! Hence Babs had to be sacrificed.
I also have a sneeky suspicion it has something to do with Jack Ashton (Rev Tom) being with Helen George in RL. He would obviously have wanted to stay in the series ( as Helen George is supposed to be returning)... and whereas in the past they have kept things open for a main character (think Chummy, Patsy...) and have let the lesser character either drift on or leave (think Sgt Noakes or Delia) this time there was no drifting for Tom and no open door left for Babs.
You watch... Tom & Trixie will be back together before you can say 'Campari anyone?' next season.