So that this dance doesn't have to continue, let's swiftly go through every single Lego Ideas set.
Shinkai 6500: One of the earliest models to be chosen back in the CUUSOO days, this was one of the test models. Limited release only in Japan, only 10,000 sets made, every single one of them was bought. So you could try and claim this as having a "very specific target demographic," and that's because it was. This was the test run, of course it was targeted. But even then, Lego did things like the Maersk Triple-E Container Ship outside of Ideas; it would have been a very board demographic outside of it if they went all the way.
Hayabusa: A Japanese spacecraft, not even one of the popular NASA ones, was not only made into a set back in the old CUUSOO days, but was the first one to be released internationally. I think that says a lot about space sets and why so many of them continue to get made. And it only came out a year after the Shinkai 65000, this site was still in its testing phase.
Micro World: It's Minecraft. I don't need to give a reason.
Back to the Future Time Machine: Now this is an important one. Yes, big famous blockbuster trilogy, tons of people have seen or know the classic. But this is the one where Lego likely realized that not only was it viable to sell products of properties that were dead (nothing new was being produced for it), but that the IP owners were happy to have Lego do stuff with it.
Mars Science Laboratory Curiosity Rover: Now NASA based sets go way back to at least 1995 with the Launch Command line, 2003 with the Discovery Kids channel, 2011 with Space Shuttle Endeavour, and many more as we see down the line. NASA and IRL outer space is a powerhouse of a market. As a matter of fact, not too long ago Lego Ideas held a vote for one of the rejected projects to be brought back and turned into a set. And it was another NASA set.......partially because NASA told everyone on Twitter to vote for it. Though so I don't have to repeat myself, group NASA Apollo Saturn V with this.
Ghostbusters Ecto-1: Similar to the Back To The Future set, but this time a new movie was being produced. A generation changing classic, heavily beloved and known by millions. Can't have as easy of an internet outrage if that was not the case.
Exo Suit: This one I would say has the smallest demographic. It's a very specific style of hardsuit/exo-suit that only a small, but dedicated group of MOCists tend to build. And you can see how much Lego changed it to make it appeal to a broader audience. Adding bright green figures and making them like Classic Space to grab the heart strings of older fans. Added a cute robot turtle that kids and moms alike would love. And a more kid friendly mech, because lots of people love mechs. And even though this is the most MOC-like of the bunch, Lego already had the rights to it. It appeared in a Lego book and in one of the Master Builder Academy sets prior to becoming an official set in Lego Ideas. Basically, Lego already had it in their hands and advertised it to tons of kids.
Research Institute: It's basically a minifigure pack. A minifigure pack encouraging women in science and has small vignettes that would be perfect for any Lego City. Really, it's basically another City set, and those sell very well. And when it also promotes the trendy social thinking, how can one resist making all the money?
Birds: People love birds.
The Big Bang Theory: One of, if not the largest, and most watch sitcom of all time. Not just within the US, but across the entire globe. It is a powerhouse of a property that has managed to create a very successful spin-off. This is a property scammers have been taking advantage of on Ebay and other sites for years. People want their favorite characters in Minifigure form, especially something as large as this.
WALL•E: Beloved Pixar film. Do I really have to explain it?
Doctor Who: A show that's been around for as long as it has and successful as it has........you can't say it's a small demographic. Specially with how synonymous it has become with nerd culture.
Maze: Like all those other maze puzzle games you see constantly in stores, but now Lego.
The Beatles Yellow Submarine: It would be like making a set of Elvis or Michael Jackson. Everyone has heard of them, everyone knows of them, no-one will shut up about them. If it didn't make bank, that would be the surprise.
Caterham Seven 620R: Fun fact, the two most common Lego Ideas projects are car and train models. When Lego started that rule where you couldn't do properties they already held the license for, almost all car projects vanished. People love cars, they love model cars, and this is a very nice and popular car.
Adventure Time: Super popular cartoon show.
Old Fishing Store: Generic appeal as with City and Modular Building sets, but one Lego didn't think of doing yet.
Voltron: Now if it wasn't for internal bias at Lego, you could dismiss this one. It is one of the smaller demographics; most people know it about as well as they know He-Man and Thunder Cats. A good number to be sure, but not huge. But then that Netflix show happened and suddenly it was in vogue again. Even though its of the original version, now everyone can have Voltron in Lego. That and its a colorful mech made of cats, there's a reason why this property was still remembered and brought back.
Women of NASA: I separated this one from the rover for a main reason. Aside from the NASA reasons, there is a political agenda behind this as much as a historical one. It's not only giving proper credit to individual who are otherwise ignored or not talked about as much, it's putting them front and center of the spotlight. And with a similar films coming out around that time about that same subject, there was a ton of marketing buzz for it. People wanted this representation in their Lego, NASA fan, history fan, or otherwise.
Ship in a Bottle: One of the coolest things to make, but now in Lego. Everyone likes impossible bottles and now they can do it in Lego.
Tron Legacy: Another one with a smaller demographic. Lots of people know Tron, lots of people reference Tron,and it has a cult following.......But, yeah, definitely one of the smaller ones. Honestly, the only thing helping it would be the popularity of Kingdom Hearts and its appearance in those games. But even though it's smaller, it's still a much larger following than Bionicle. Because Disney.
Pop-Up Book: It's in the name, it's an item people already love, but in Lego.
The Flintstones: Before the Simpsons took the title, it was The Flintstones that was the longest running and most successful animated TV show. Especially one with a more adult-target demographic. And they are still so popular that there's successful cereal brands with them.
Steamboat Willie: The sole reason why public domain laws keep getting pushed back. If that doesn't tell you of its notoriety and power, I don't know what will.
Treehouse: Lego somehow hasn't done this yet. It's most children's dreams to have a treehouse. Now in Lego form to print money.
Central Perk: Before Big Bang took its title (and it can be argued that it hasn't still), Friends was the undisputed king of sitcoms. When the show ended, it continue to run as reruns and basically was not off the air until streaming services like Netflix came along. The ratings and amount of views it in the States alone, not to mention world wide, is thought to spell doom for whatever platform is unable to keep it.
Dinosaur Fossils: Dinosaurs, but with more spooks.
Honestly, you would have an easier time arguing for Idea Projects that had a huge demographic and were rejected. They turned down plenty of bankable projects, even ones that didn't require a license.