Well it seems like one of the main reasons, there will be no season 6, is because our king Arthur decided he is done with Merlin.

He explains that he has known for quite some time that Merlin will be ending with season 5. Which leaves us to the question;then why did they lead us on about a possible 6th series?

Chances are that even at the comic con in july, where the possibility to a 6th series was announced, Bradley already knew he did not want to do another season.

I have great respect for Bradley coming out with the truth, but I am pretty disappointed to find it out this way.

Source: http://www.digitalspy.co.uk/tv/s53/merlin/interviews/a442466/merlin-bradley-james-qa-the-final-episodes-will-surprise-fans.html