...don't understand is how come TV show writers don't resolve plot threads.

There are a whole bunch of plotholes everywhere I look, and although some of them are there intentionally to increase the hilarity of shows, a multitude of them are just plain frustrating.
1. My Name Is Earl

This show was pretty popular while it was still running, mostly because it had an original storyline to it. It was about a dumbass who realized that he needed to change every single bad thing he did in his life. 

The whole series had four seasons when it suddenly got cancelled, and it left every one of us with a particularly interesting question still unresolved. Yes, I'm talking about Earl Jr.'s father - as in, who was it? 

2. 100 Deeds For Eddie McDowd 

All right, since I mentioned My Name is Earl, I couldn't leave this one out. This particular series was kind of a modern Neils Holgersson, only in this story the kid is not shrunk - he is turned into a dog. As you probably already guessed it, the whole show was supposed to have 100 good deeds so the guy could turn back into his original form, only the show once again got "mysteriously" cancelled, and we never found out what really happened. 

Though I did hear about a possible scene where the dog was taken to the pound and put to death. Hah... joking. That would have been a good move from Disney. 

3. Carnivale

This was a cruel joke, I tell you! Carnivale had written material for about six more seasons when it got cancelled, and so it's not a surprise that we were left hanging.

And we were left hanging at the worst possible place, with Ben wounded and unconscious.

4. Lost 

The king of unresolved plotlines, Lost forever entered the top list of disappointing TV shows (and it would be a miracle if something would push it out of its place). 

I'm not going to waist your time with explaining what each of this plotlines covered in case you didn't watch the series (and in case you did), but I do have a list: 

1. What the hell was the deal with Cindy? 
2. What's with Hurley's curse? 
2. What was the deal with the Tampa Job? 
4. Why was Libby in Australia? 
5. What is the Basra Incident? 
6. Why did Walt want to leave the island? 
7. Did Ben kill Henry Gale? 
8. Why does Jacob's cabin appear to Hurley, and why does it shift around? 
9. What's with Mikhail's eye, why didn't he die when he was hit with a harpoon and where did that grenade come from? 
10. Where is Christians's body?
11. What is the business with Daniel? 
12. Why can Desmond survive electromagnetism? 
13. Why was the orientation film edited? 
14. Where does The Staff go, and why does it have a secret room? 
15. Why can't the smoke monster cross lines of gray ash? 
16. Why does the smoke monster make mechanical sounds?
17. What caused Walt's powers? 
18. What was the deal with the Ajira massacre?
19. What was the deal with the Mother (the protector of the island before Jacob)? How did she end up there in the first place?
20. Why are certain characters mediums?
21. What was the light source on the island?