There is such feature already. It's called a save.
Yes true, but lets face it, a feature that lets you cancel quests would be nice. It would help people with bugs, people who don't save alot, or people who want to clear clutered quest list, and many others.
Also, to my understanding ( I might be wrong ) but lets say you have a quest thats really important to you and its bugged and you have a save before you do it. Wouldnt just going to the save and restarting it just end up with the bug again? So perhaps the cancel and restart components help agenst bugs? Again, I just thinking, I do not know if I'm right or even close.