in robin hood story marian didn`t die so why in this story she died?

Apparently it was because Lucy Griffiths wanted to leave to try other roles and go to Hollywood. Not sure if its ever been confirmed though.

well because obviosly the actor wanted to leave th showso they killed her off

I saw an interview with the two creators, Foz Allan and Dominic Minghela. They said that Robin had always had Marian, and they wanted to see what the hero would be like without her. It was more of an expirement, from what I understood.