My theory essentially boils down to the proposition that MIB somehow picked up the ability manipulate time (through electromagnetism which will be explained later) and it all had to do with his plan to use Locke (and eventually his likeness) and Ben to kill Jacob. He made all of the candidates (that he could not kill) jump through time while he executed his plan (more below).
I have been thinking a lot about why certain castaways were sent into the 1970's and while others remained in the 2000's. This is one question that I anticipated would be answered one way or another as the season progressed. I am not much of an "Answer" man so I wasn't that devastated (and loved the finale) but I wanted to figure it out. I apologize if there have been similar theories in the past but I needed to get this off my chest as I will listening to an old podcast that inspired me to think about this.
II. THE CANDIDATES JUMP THROUGH TIME
First, I want to look at the list of candidates:
As we know, MIB could not kill these candidates. Instead, however, he used what he "could" do (i.e. manipulate time, people) to keep the candidates away from the modern timeline so that they could not interfere with his plan.
First, Locke began jumping through time for a purpose separate and apart from the other candidates. He needed to use him to manipulate Ben as had been his plan all along. For Locke, the time travel was all a part of MIB's elaborate plan to convince Locke of the island's specialness (which MIB ironically denies...)
This has probably been discussed but at least a few of the trips in time was part of MIB's plan. In the 50's, it was all part of his plan for Richard to be persuaded that Locke is special. A couple of the trips were to orchestrate Locke getting trapped under the "former" well to meet with Christian (by jumping from a post-well time to a pre-well time). This has been discussed a lot so I won't go into too much detail. Suffice it to say, the purpose of Locke jumping through time was to trigger his eventual death by the hands of Ben (to be subsequently reincarnated to bid more dirty deeds out of Mr. Linus).
*Side Note/Footnote: Christian's presence in pre-Well times is further proof that MIB can manipulate time. How else would Christian (whose body did not arrive on the island until 2004) be there in the early times of the island (via MIB)?
Footnote 2: It could be easy to nitpick here and say: "Then why didn't he just go back in time and save his mother from his other mother? Why didn't he just go back in time and do X, Y or Z?" I don't know. I guess if he couldn't kill Jacob and couldn't kill the candidates, what was the point? Or, conversely, maybe his ability to time travel was controlled by his electromagnetism and he thus could only travel to certain times etc. Maybe he was using his own electromagnetism and the island to travel himself and worked hard to fit his plan within the confines of when and where he could jump to (more on that AND JUGHEAD later).
II(b): THE OTHER FIVE CANDIDATES
Then there is everyone else who flashed through time. MIB needed to keep these special people away while he executed his plan. He couldn't kill them and they could potentially get in his way.
First Sawyer was catapulted through time and landed in the 70's. Jack, Hurley and Sayid followed with a trip to the 70's via Ajira Airways. Of course, we all know that Kwon could mean Jin or Sun (or even Ji Yeon). However, maybe this is Damon and Carlton's proof (to us) that Jin was the 6th candidate and MIB, despite saying he doesn't know which Kwon it is, knew all along and made sure Jin was back in the 70's. The bottom line is, assuming Jin is a candidate, all six candidates were sent through time (and away from Jack's plan).
III. WHY WOULD JACOB BRING THE CANDIDATES BACK?
So why did our candidates COME BACK to the modern timeline? My answer is that MIB's plan was two-fold: (1) Kill Jacob (with the candidates MIA or MIT ("missing in time")) and (2) After Jacob is done, bring back the candidates so that he can figure out a way to have them kill each other. After Jacob was dead, he needed to get the candidates back to the 21st century so he could take care of them and get off the island (or destroy it or whatever his purpose was...)
This brings me to my point on Jughead. Is it possible that Jughead in the 70's was all part of MIB's plan to get the candidates back to the 2000's? MIB's electromagnetism combined with the island allowed him to jump through time and the electromagnetism of Jughead had the same effect on the 1970's Losties?
To elaborate, MIB could somehow jump to different times but he always managed to return to the 2000's. Maybe he was able to return to the 2000's because he himself was electromagnetic and the Losties had to use Jughead to essentially become electromagnetic and return to their appropriate time period (explaining why an impaled Phil didn't join them on their journey...).
Now when talking about electromagnetism and time travel there will be holes. Obviously it is not perfect but it makes sense to me. As an aside, this is on the problems I have with the people (who I agree with on some questions) who want answers to EVERY question. There is no good way to answer a silly question. I mean it felt a little weird when we were all of a sudden at a magical cave with a light in an effort to explain the smoke monster. I apologize for the digression.
IV. WHAT ABOUT THE OTHERS (Miles etc.) WHO JUMPED THROUGH TIME?
This is a tough one. In addition to Sawyer: Juliet, Daniel, Charlotte and Miles also jumped through time. My only guess is that these people each served a purpose of making sure the candidates (or anyone else) did interfere with his plan.
I'll go in order of my confidence in these guesses.
Miles: First, it is possible that MIB was threatened by Miles' ability to talk to the dead. Imagine Miles standing on the beach and walking by that crate with Locke's REAL body in it. Locke's pre-Dead self would say something (i.e. "Why is Ben killing me") and blow Locke's cover.
Faraday: Maybe MIB was threatened by his brain. Or, another idea I had is that Faraday was sent back to get killed by his mother so that MIB could eventually use Eloise and Widmore in the future by playing upon the emotions (or anything) related to Eloise shooting down their son.
Juliet: Maybe Juliet was a part of keeping Sawyer happy in the 70's so that he could follow through on his plan. After all, you have NO IDEA what he did to get to that point of killing Jacob.
Charlotte: Who knows. Maybe it had to do with Faraday. Maybe MIB was threatened by her smarts as well. This is tough one.
Rose, Benard, Frogurt and the others: No clue. Maybe he didn't want Sawyer and company to feel special. I'll stop thinking up silly reasons for her presence in the 70's before I get comments about a crackpot theory that was one sentence in a post that is most certainly not about Frogurt...
All I can say is it's a shame Lapidus didn't end up in the 70's...that would have been fun!
I guess the moral of the story is: It's a shame Jacob was so hands-off and let this all go down...
I am sure there are some holes in my theory and the fact that I am yet to do a re-watch of Season Five with Series Finale eyes makes me ripe for criticism. But I welcome it. I'd love to learn more about this conundrum. I know it has to be more than "The Writers just like time travel." I think the writers implicitly answered a lot of questions and we will come to see that with the re-watch.
That being said, I look forward to a re-watch with what we know now so maybe I can refine (or APOLOGIZE) for my theory in the future.
Thanks for getting through my ramblings!