Firstly it wasn't their only campaign promise. Secondly they didn't win so had no mandate. Thirdly it was a coalition and the Tories had a slightly stronger man

Firstly it wasn't their only campaign promise. Secondly they didn't win so had no mandate. Thirdly it was a coalition and the Tories had a slightly stronger mandate. Finally they had to compromise on some things to achieve other arguably more important goals. This is how politics works. If you don't get this then I don't know what to tell you.