The CW has created a small empire of DC Comics-based shows, and the connected universe comprised of Arrow, The Flash, and Legends of Tomorrow has been a source of all kinds of superhero entertainment over the years. Supergirl joined The CW in Fall 2016, but it still remains separate from the other three series as it takes place in a different in a different dimension. Arrow, Flash, and Legends of Tomorrow are all set on Earth-1 while Supergirl is set on Earth-38. But is that necessarily a bad thing? Not really.
Supergirl taking place on Earth-38 has meant that crossovers to other DC TV shows aren't quite as simple as hopping the train to another city, and 2016 afforded a couple of big opportunities to combine universes and simply incorporate Supergirl into the Arrow-verse. But it didn't expressly happen, and here are 7 reasons why Supergirl is best left on its own on Earth-38.
Alien Threats Don't Fit In The Flarrow-verse
Each DC TV show pits its heroes against a certain kind of antagonist. Oliver fights human villains on Arrow, Barry battles metahumans on The Flash, and the Legends combat timeline-related threats on their show. But Kara Danvers battles aliens, and most of her bad guys are so powerful and nearly unstoppable to the Girl of Steel herself that they could squash the heroes of Earth-1 without too much trouble.
The only time that the good guys of the Flarrow-verse fought outer-planetary foes was in the "Heroes v. Aliens" crossover when the Dominators arrived on Earth-1, and it took all of their combined forces and an assist from Supergirl to emerge victorious. Supergirl's aliens are just too big to fit into the same world where Oliver Queen's biggest foe is another archer.
Kara Is Simply Too Powerful
Just as the alien villains of Earth-38 are too formidable for the heroes of Earth-1, Kara Danvers is too powerful as Supergirl to live in a world that has been getting by with the likes of Oliver Queen, Barry Allen, and a crew of time-traveling misfits as its protectors. Green Arrow, The Flash, and the Legends are great characters who are exactly what their world needs, but Kara can pretty much do everything that they can do and then some. She has superspeed, invulnerability, flight, laser vision, freeze breath, super-hearing, super strength, and x-ray vision.
If she lived on Earth-1 on a regular basis, we'd have to wonder why Kara isn't constantly being called up for an assist. Honestly, Kara and the D.E.O. probably could have taken out the Dominator threat themselves with no need of a crossover. She's too super.
Team Supergirl Has Everything Covered
Between Kara, her friends, and her allies, Supergirl has pretty much everything in Earth-38 that the other three shows bring to Earth-1. Kara is fast enough that she at least rivals the Flash, which means that she could certainly be capable of time travel if she runs at a certain speed. Winn is a hacker extraordinaire who can pull off the impossible, a la Felicity Smoak. Guardian is a vigilante who does his best to protect his city without superpowers, a la Oliver Queen. Alex Danvers uses her military resources to act as a hero in her own right, a la John Diggle. Everybody is covered.
Kara even works for a newspaper, a la Iris West. The similarities work because Kara and Co. are on a different Earth, but they might feel redundant if Kara and Co. join all the other heroes on Earth-1.
The Different Standards Of Reality
Earth-1 has been more or less grounded in reality ever since Arrow debuted in 2012 and established the foundation for the Flarrow-verse. Admittedly, everything has gotten a lot more fantastical ever since The Flash and Legends of Tomorrow started bringing in time travel and metahumans on a weekly basis, but Earth-1 at least started out as our Earth. The Supergirl Earth hasn't been based in our reality for a long time.
Aliens are acknowledged as members of society. Superheroes don't have to wear masks and can zoom around fighting crime in daylight. Even the President of the United States is secretly an alien (and played by Lynda Carter). Key aspects of Supergirl are reliant on its own standards of reality, and those standards can't hold up in the world of Arrow, The Flash, and Legends of Tomorrow without a lot of story-shifting.
Batman Exists On Earth-38
Supergirl already introduced the biggest DC superhero of all time with Superman, and Season 2 has established - although not explicitly - that Batman exists on Earth-38 as well. National City citizens have mentioned Gotham, and Kara herself made a reference to her super-cousin's team-up with a vigilante who has "lots of gadgets" and "lots of demons."
While Batman himself may never be able to show up on The CW, he fights crime on Earth-38, and members of the extensive Bat-family could be fantastic (if only temporary) additions to Supergirl, and Oracle and Batwoman are the kind of badass women who would fit right in on Supergirl, while Nightwing could be a lighthearted alternative to the Dark Knight. At this point, we've seen nearly 10 years of Earth-1 thanks to the Arrow flashbacks, so if Batman or other big DC heroes existed on Earth-1, we'd have heard about them by now.
The CW Probably Couldn't Pull It Off
The CW has done remarkable things with its DC TV shows, and it's likely that all the other comic series on the small screen nowadays would not exist if The CW hadn't been successful with Arrow back in 2012. That said, The CW probably can't merge Earth-38 into Earth-1 without making a mess of it. Arrow, The Flash, and Legends of Tomorrow already have to remain consistent with each other, and throwing Supergirl into the mix might never allow for a logical timeline connecting the shows ever again.
Of course, the teams behind the scenes could draw on the Crisis on Infinite Earths storyline that combined Earths in DC Comics, but the Flashpoint story on The Flash has shown that The CW lacks the budgets to adapt such huge comic stories without weakening them somewhat. Earth-1 and Earth-38 exist just fine separately; merging them could get unnecessarily messy.
The Universes Are Basically Connected Anyway
The "Heroes v. Aliens" crossover had to use Barry's speed and Cisco's vibe abilities to fetch Kara from Earth-38, and it took them a few tries to actually materialize in her world. By the end of the crossover, however, Cisco built a special inter-dimensional extrapolator to allow Kara to travel back and forth between Earth-38 and Earth-1 whenever necessary. She could even use it to communicate with her new pals on the other side.
Crossovers still won't be as simple as one character hopping the train to another city, but folks from Earth-1 and Earth-38 now have a way to meet up more often than once a year. As such, this is one more reason why there's just no need to pull Supergirl away from its own universe when it's already doing so well.