Man of Steel was intended to be a stand alone film, at most the beginning of a Superman franchise much like Batman Begins from the soon to be wrapped up Nolan Batman trilogy.
Then the Avengers happened, and the comic book movie world on both sides of the ticket... well they went nutso.
Soon Marvel was releasing a ton of information on their Phase 2 and Phase 3 projects, revealing that plans had long been in place to extend their shared universe (The Marvel Cinematic Universe or MCU) past the Avengers all the way to deep space with Guardians of the Galaxy, and then back down to the man on the street with Agents of SHIELD. They also made it clear they were hitting everything in between - even magic and Ant-Man.
Infinity Gauntlet Thrown. DC?
DC had two live action projects in the pipe at the time. Man of Steel and Arrow, and they were very different projects.
Man of Steel was the sausage that was finally getting pressed out of years of Warner's attempt to bring Superman back to the big screen after the tepid reaction to Superman Returns. The directors' version of spin the bottle landed on Zack Snyder, who’s Watchmen had been a box office success for WB and a mixed but generally positive critical reaction for Snyder's take on the long awaited adaptation.
It wasn't a sure thing, many directors had passed on the seemingly impossible project, and Snyder won out by more or less being the last man standing who wanted the job. His most recent outing, Sucker Punch, receive one of the most polarizing receptions of any movie in recent memory - some hailing it as a brilliant feminist film, others as cheap female exploitation. A foreshadow of what was to come with MoS.
Man of Steel’s box office success quickly green lighted a sequel, and the sequel was recently announced to be a Batman vs. Superman team-up/smack down. Slowly but surely, the pieces of the DCCU are coming together on the big screen. Slowly being the key word here. By the time MoS2 (or BvS, or World’s Finest – whatever it will eventually be called) comes out, there will be up to FIVE Marvel films made in the MCU alone, including Avengers 2. This doesn’t even count Sony and Fox running Spideman, Fantastic Four, and X-Men movies at the same time.
This, coupled with the near paralysis of Warner’s to commit a film to another character besides Batman or Superman, means that they are significantly behind Marvel.
Warner's reaction to the Avengers was... lacking. First they announced Christopher Nolan as executive producer even after he long ago left the project to focus on his final Bat film. This with the earlier decision to have David S. Goyer to write the script for the new Superman film suddenly looked like Warner's were sticking with what was comfortable, not necessarily what was best for the Superman character, or the DC universe at large.
Now Man of Steel not only had to bear the weight of rebooting a franchise that had long languished in development hell, it also had the additional responsibility of performing well enough to establish a shared DC Cinematic Universe (DCCU). Not only that, WB was also taking a wait and see approach to MoS's box office before deciding to move on further projects. Leaving everything related to the DCCU in a lurch until the first BO numbers came in.
Meanwhile, the other DC project was Arrow, a live action TV series based on a character practically unknown to the general public - Green Arrow. Expectations about the success of this series were low at best, so much so that they threw out most of the continuity from the comics and even dropped the "Green" because of the poor connotations to the prior year's Green Arrow and Green Hornet flops.
Then, something kind of amazing happened - Much too literally everyone's surprise, Arrow became a hit. Changes to the mythology and strong writing and performances brought a tier 3 character accused of being a cheap Batman knockoff into his own.
Some speculation emerged that Arrow could be rolled up into the larger DCCU, but there were a number of issue with this. One being that Arrow is decidedly based in total reality, to the point where comic book super powers are turned into technologically based tools and weapons - which gets difficult when you are trying to include the likes of Wonder Woman, Aquaman, and Martian Manhunter in your mythology.
Also, with a crowded roster already, there wasn't likely going to be room for Oliver in a Justice League film, whose function on the team is largely overshadowed by Batman in the first place.
But then...
Then The Flash announcement happened. It was revealed that no less than Barry Allen would be showing up in Star(ling) City in Arrow's second season as a backdoor pilot to his own spin-off series.
Barry Allen's The Flash cannot be explained away by technology, he is as superpowered a hero as they come. His mythology is deep, and most importantly - unlike Green Arrow, a lot of people care about The Flash, and in particular Barry Allen's turn with the mantel.
This represents a major shift in the tone of the Arrow-verse, and could mean that DC is setting up for TV and Film crossovers. If this is the case, then it may be the first move DC has made since the success of the Avengers that might actually propel them near Marvel's level in the live-action superhero arms race.
A quick word about TV versus Film. It's often assumed that TV is the lesser of the two mediums, and now more than ever, this simply is false.
First, look at how well movies flow into TV series. Buffy the Vampire Slayer, MASH, Friday Night Lights, Alice (Doesn't Live Here Anymore) were all films that spun off into highly popular and critically acclaimed TV series. The same cannot be said for TV shows which spawn movies. Twin Peaks, The Simpsons, Firefly... all of the film versions of these very popular shows feel either like extraneous extensions of a concluded story, or merely a longer and better looking episode of the show.
This is because TV has TIME and films have BUDGETS. TV can craft their stories over years, leave loose ends open for long times, make course corrections, etc. Even a single season series like Firefly has the bulk hours to tell a number of nuanced stories. Films mostly do not have these luxuries as the money involved and the short running time should mean that a film has to hit hard and fast and keep everything moving. There is little time to develop character, particularly if you spend significant time on the gee-wiz moments that make up the bulk of a superhero movie.
That said, TV is actually the perfect format to bring The Flash to the public eye, and Arrow is a perfect place to launch it from. The bright, friendly, and hell FUN dynamic of Allen's Flash is a major contrast from the dark and brooding Queen. A team up on Arrow is a perfect way to show this contrast, and also a great way to interject a little fun into the perpetually dark and gritty DC live action universe.
In addition, The Flash has an incredible range of powers, much more than just “the guy who can run fast”. However, a lot of these powers aren’t going to be immediately obvious to Barry, and TV gives him the time to explore what he is capable of. Also, The Flash has one hell of a rogue’s gallery – second only to Batman’s. They often team up and just go by “The Rogues”, and TV has the time to explore each individually, and as a team.
The real key here is that The Flash is a mainstay of The Justice League, and he has a story arc that requires time with the character to get to know him and his motivations. Bruce Wayne said that, if his parents hadn’t been killed, he hope that he would be the kind of man Barry Allen is. This is someone we need to get to know on a deeply personal level before we let him lose on a JL team up. I don’t think a single film could do that, but a show… defiantly could.