I don't think they had a choice if memory serves correct. Warren & Murray wanted to do other things and they're not the only ones who have tried to use the program to progress their careers. Murray was briefly on Dexter. Annoyingly she was cack and fugly topless, which was odd since she played the sexy character in Hustle really well.
There was chemistry present between them in the first four series and like any good relative unknowns they certainly took their chances. But it was really the scripts that made Hustle in my opinion, with heavy emphasis on good direction.
When they both left, the quality of the story went down at the same time I felt. There was also one episode where they either repeated the main trick or something; everybody predicted it after a sense of deja vu before it "happened" on the first broadcast.
Just seen the latest one as well. Didn't make that much sense and was too far fetched even within their universe. The new "fit" bird is just awful. I don't think they should do another series.