Why are The Simpsons (TV series) so apparently successful in "predicting" the future? It has been widely commented in the yellow and not so yellow press that The Simpsons (the TV series) have repeatedly predicted the future. Comprehensive online articles about it are here and here. If you google "simpsons predict the future" you get millions of hits and videos. 
Perhaps the most remarkable "prediction" (at least to me) is Trump as US president (made in 2000!). The latest seems to be the legalisation of Cannabis in Canada.
The question is, why this apparent success?
My guess is that, (i) The Simpsons make a lot of "predictions" (rather, scenario building), (ii) statistically speaking, the proportion of hits is actually very low (not that I have compute it). The apparent "success" is just a cognitive bias. 
 A: 
Perhaps the most remarkable "prediction" ... is Trump as US president (made in 2000!).

That is indeed a pretty impressive prediction, but it is less outlandish than you are probably aware.  In regard to this prediction, it is worth bearing in mind that even by 2000, Trump had established himself as a popular business figure in the USA, and had made well-known forays in entertainment and politics, including an aborted presidential run.  During the 1980-90s Trump had made regular forays into political issues and published various newspaper advertisements setting out his views on foreign policy and crime control.  In 1999 he sought candidacy as the Presidential nominee for the Reform Party of the USA, but he withdrew from his attempted candidacy in February 2000, citing problems with the party.  He indicated that he might run for President in a future election.
Trump was one of the most admired business figures of the 1980-90s.  He had made a successful career in New-York real-estate and also commonly featured in popular culture (e.g., in a bit part as himself in the 1992 movie Home Alone II).  He was regularly interviewed in the media in relation to political and social issues in New York.  In September 1987 he published a major advertisement in multiple newspapers advertising his foreign policy views (see e.g., this NYT article).  His spokesman told the media, "There is absolutely no plan to run for mayor, governor or United States senator.  He will not comment about the Presidency."  In 1989, during a period of high crime in New York, he published another advertisement calling for reinstatement of the death penalty and increases in police.  In a 1989 Gallup poll he was listed as the tenth most admired man in America.
Aside from The Simpsons there have been various other items of popular entertainment that featured Trump in his aspirations for the presidency at around the same time.  The video clip for the 1999 Rage Against the Machine song "Sleep Now in the Fire" shows the band holding a concert on Wall Street, with a mixture of head-bangers and bankers, and they show one of the bankers holding up a sign promoting Trump's 2000 presidential run (at 1:04 min).
The episode of The Simpsons that you are referring to would probably have been written during the run-up to the 2000 presidential election, and so the writers would have been aware that Trump was a candidate for a minor party.  It is likely that the episode was making fun of the fact that he was (at that stage) an outside candidate for a minor party, who had some populist support, but was unlikely to win.  Trump's aborted candidacy for President in 2000 foreshadowed his later runs, and even when he withdrew from that attempt, he indicated that he might run again.  In January 2000, prior to withdrawing from his presidential run, he released his political book, "The America We Deserve".
I suspect that this episode of The Simpsons was just poking a bit of fun at a possible future with a populist candidate who was running for a small party.  However, even back then, it was not a stretch to imagine that Trump would run again for the presidency.  It is certainly an impressive prediction, but less so if you know the history of Trump in politics.
A: Quick thoughts:
Let's pretend—instead of scenario-building or making jokes—each of their plot lines are actual predictions.


*

*They make a lot of predictions, so their Type I error is very high, but their Type II error is also very low. If every creative choice is making a prediction AND they have been around for decades, then their show is similar to a medical test that almost always says you have whatever disease you are testing for: You will almost never miss a positive case, but you will be telling a lot of people that they have a disease which they do not have.

*People probably only consider a subset of scenarios ("predictions") that are feasible. If the Simpsons were visited by aliens, nobody would consider this a prediction—because we know the odds of it happening are very low. So the universe of predictions we are considering is highly correlated with the prior probability that they will come true—this is stacking the deck in favor of the Simpsons.

*The writers of the Simpsons are smart people that also live in the same society in which they are making their "predictions." They are trying to be funny, so what they do, in a Bayesian sense, is construct funny situations that are not assuredly going to happen (these are boring predictions) and are not never going to happen (these are absurd predictions). So again we see the prior probability of these things happening stacking the deck toward the Simpsons being correct: If they write about things with a solid-ish probability of happening (like Canada legalizing weed), then we shouldn't be too surprised when their predictions are correct.

*There is no time limit on their predictions. This gives us an unlimited amount of "trials" (let's say the unit of analysis is days or elections or news cycles or celebrity careers, etc.), and all we ever need to do is hit truth once and the Simpsons are "correct."
When you consider all of these together, we can see that making a ton of predictions over an unlimited number of trials where you only have to hit once to be "correct," and people define "success" by ignoring Type I errors and shaping the universe of possible as things with only some probability of occurring, and the creators themselves generally make predictions in areas that have some prior probability of occurring—we can get to the conclusion that the Simpsons can "predict the future."
