I have watched every episode of The Flash (2014) and I can't understand or remember any of actors explaining: why hasn't everybody become a metahuman?
Was there ever an explanation given?
P.S. : I won't read any comics... So feel free to spoil the story!
The new Flash series created a circumstance by which they could explain metahuman capacity (the partial destruction of the particle accelerator) with a random element, (the dark energy wave) and using it to create a variety of metahumans who existed in the comics and often gained their powers through accident or scientific interactions.
The writers used this event to create metahumans whose origins occurred under a variety of different circumstances and for a variety of different heroes. By creating a centralized event, they have an opportunity to cherry pick who will get metahuman abilities and a centralized (mysterious, unknowable) source for them.
The Weather Wizard, for example, was an old Flash foe who originally had his powers through the use of technology (his wand), while Plastique started her career as an enemy of Firestorm. Girder was a Flash foe with a very different appearance, and Blackout was a creation of the Flashpoint alternate universe and was a "good guy" during that event. Now with the creation of this singular event, separate origins don't have to be created for each of them and we can start the story right away.
This is a problem of both the comic medium and the conversion to the televised medium. They don't want new viewers to have to know anything about the history of the character. So the event which creates the Flash and his enemies has to be something simple enough for viewers to relate to but not have to work too hard to understand.
The Flash is a metahuman whose staggering powers need unusual, talented and most often metahuman villains for the Flash to play against. Something this version of the show has managed to do well is to portray the metahuman aspect of the Flash's life and the Human aspects of Barry's life in equal proportion.
This new Flash also seems poised to seed the TV DC universe with metahumans unlike the Arrow and Batman series which focused mostly on highly trained or technology-assisted humans for the most part. If you're a comic reader you have already noted the creation of Captain Cold's Cold Gun, and Heatwave's fire gun.
There are also homages (or even potential people who will later develop powers -- not likely but possible) including Cisco Ramon (know as the metahuman, Vibe in the comics), Dr. Caitlin Snow, (in the comics, she is the villain, Killer Frost) and the recent debut of what looks to be Ronny Raymond as Firestorm. We got a glimpse of General Wade Eiling as well...
Carlos Valdes as Cisco Ramon (Vibe) and Danielle Panabaker as Dr. Caitlin Snow (Killer Frost)
Speaking on a more scientific level, I would say for the same reasons that not everyone who is exposed to carcinogens will get cancer, and not everyone who is exposed to various viruses will catch said viruses.
Special circumstances were in play for those who became metahuman during the accident. Barry was bathed in certain chemicals when the dark matter and lightning hit him. Any other man quite probably would have died, but he lived. Why? Girder should have died when he fell into the molten iron. Firestorm should have been incinerated. Etc, etc.
My theory is that each of these people had some aberration in their DNA that allowed them to survive these events and the dark matter combined with the particle wave altered their bodies to give them the powers.
Disclaimer: I never read the comics either.
This is not addressed (yet?) in the TV shows. It may be possible that everybody in Central City is a metahuman to some extent, but only a few persons exhibit special abilities.
Being a metahuman is an alteration of the genetic structure due to the exposure to particles released during the explosion of the StarLab particle accelerator, hence results vary from people to people. Note that in genetics, a mutation may not have visible effect.
Also, note that the effect of the alteration depends on the context where the meta-human was at the time of the StarLab accident. For example,
While the accepted answer does a great job explaining it out of universe, I think this has been explained in universe in the very first episode, explaining "pockets" of dark (or exotic I don't exactly remember) matter were dispersed undeterministically over Central City without being a constantly distributed event.
That is, as they explained it, I seem to recall those pockets were scattered as small dots in a map, with only a small radious of effect being exposed for a given length of time. Metahumans were humans that happen to be in the right (or wrong) place at the right moment but it wasn't a city wide effect.
To make a comparison, it was like dropping hundred of thousands of "paint bombs" from several planes over the city. The affected area is huge, but it's not the whole city and you have to be on the right place to have one of them fall right on top of you and become a metahuman.
In the DC comics, people who get powers posses a "metagene", a chromosomal anomaly that allows them to (potentially) survive great danger by rapidly developing abilities which either allow them to survive the dangerous event, or in the case of energy-based events, absorb or metabolize the energy.
It's not a guarantee that having the metagene will cause one to obtain powers, as one may never be exposed to a dangerous enough event to trigger it. Similarly, several people exposed to the same danger may exhibit different powers afterwards.
The explosion of Wells' accelerator is an example of I refer to as a "white event" - a large event that provides the impetus for many people to spontaneously exhibit powers in a comic book world. It's the general source of the energy required to trigger the metagene, but as explained, would affects different people differently.
While the metagene has no been explicitly mentioned on the show, it may still be a concept in play.
I've read in somewhere that only the ones who had the "metagene" and were exposed developed the meta-human abilities. I don't know If It's true, but Its a good argument and It would leave a lot of options to the writers.
Transformation on a physical level also required a certain degree of stress (Ie. Barry and being struck by lightning. Ronnie and his death. The clyde's death by weather and so on. The guy that said everyone in central city was a metahuman to some extent was correct.
