Why do some huts not give quests?

Is it because

1)  You have an open quest identical to the one the hut would give

2)  There is a limit to the amount of unfinished quests you can have total?

3)  There is a limit to the amount of unfinished quests that one hero can start total?

4)  Some other limit.

And finally, how do you cancel a quest (potion lab on a distant island, or a fetch quest deep inside territory of a neutral faction you don't want to go to war with.)

Thank you.