I could see why a handful of people, if not the majority would resort to ex-military. It works well with the lore and it's a good way to pair your proficiency in shooting in game with an IC reason as well.
Some may simply find civilian characters to be not as amusing as ex-military, simply because the game and the setting we're all playing in is based around a whole bunch of threats and even more guns. I would believe at this point it may seem like a lot of characters have a militant background, but it may just be as @Ramon stated, a lot of characters who didn't have a military background simply adapted to their surroundings and may come across as such in interactions. Good on everyone though, I'm excited to interact any kind of characters, I'm sure a fair amount of thought and creativity went into each and every one of them.