Yes to everything Gizmo said. Don't disagree with Nefla either.
But Gizmo's got the right idea imo. I mean if you want to tell a story, you need to create certain structures that by definition restrict ultimate character freedom.
Fallout 4 deliberately switches the formula up in order to convey a certain story that we know nothing about yet.
What we do know is this: It's all over. It's all over but the crying. Two hundred years of ice between the PC and the past. Emerging from Vault 111 is the starting point into a new life. The Fallout after the end of our old life. Will this new chapter offer some new insights and challenge the fundamental beliefs and convictions of our protagonist? You bet, kid.
Back to the point: Why the need to restrict character concepts? Well, honestly you don't. There's a million possibilities no matter the circumstances.
But why no possible selection of other archetypes than the heterosixual married ex-military parent from the suburbs?
REASON NUMBER 1: DISILLUSION
The Fallout world of 2077 is characterized by nationalism, paranoia and ignorance. The United States of America, while rapidly amassing wealth, widows (there have been some women serving too btw) and technological advancements during the ongoing war, are plagued by public uprisings, corruption, megalomania and internal power-struggles - in nearly all societal realms. The typical middle-class civilians live in blissful ignorance and blind patriotism, lulled and drunken from the potions of mass media (largely radio, but TV too) propaganda.
You want to know what the very first thing was we heard and saw in Fallout (after logos)? Yeah, https://www.youtube.com/watch?v=Dquq8DABdw8.
They wanted to show people who do not suffer persecution, discrimination or anything negative about the system in their lives. They wanted to show people who live in complete illusion and are to be disillusioned completely.
Now, since you're smart, how does the whole ex-military (is it confirmed?) stuff fit? Surely the footage from Fallout's intro doesn't look like one would remain an illusioned, naive, good-hearted and patriotic person after some field experience, or hell even after watching the news and giving the slightest [censored]. Well, what the character would have seen would surely be up to us anyway (I don't think it's going to be explained in detail) but think about this:
What Fallout tries to tell is that the pre-war culture is something unique. Many believe that it's just the whole 50s thing, but it's not only the soundtrack you hear, it's also the images we see from the old world. They showed an execution on public tv and they used that as propaganda. That was not inteded as a message to terrorize (well, in a way it was) but to glorify America. You need a certain cultural standard to make that in any way a promising attempt.
Surely not everyone would have been tricked. But there are some who bought into this kind of thinking and have made a culture out of it. The illusioned ones, the lucky ones, the ones who benefit from the system.
They aren't guiltless. But they are the ones who will experience ultimate disillusionment. Fallout 4 wants to capture the essence of the Ink Spot's music by making the narrative thread resonate to it in the most fundamental way, by involving everything our protagonist probably believed in.
REASON NUMBER 2: VAULT 111
Any American citizen was allowed to buy a ticket to a brighter future underground. Not everyone could afford it though.
Vault-Tec (while recieving hefty funds by the government - possibly secret additions by the secret gov) wanted the Dollar. I don't know how much the entry cost and I'm aware that many people didn't bother to even give buying a ticket a thought (which would normally lead to price decrease). I assume that good earning to wealthy people would nontheless certainly be the majority of purchasers (unless there was a certain experiment involved), especially considering they would be the major, most gullible recipients of propaganda such as http://www.tannerhiggin.com/wp-content/uploads/2012/01/VaultTec.jpg.
Maybe Vault 111 involved an experiment that has to do with your military past? In either case, depicting the protagonist as a middle-class, robot-owning, family raising, ex-military suburbanist was a good decision in order to convey financial independence.
Additionally, the army background is perhaps the most earning one when it comes to logically explaining your later proficiencies anyway. So it's quite smooth.
REASON NUMBER 3: STILL A LOT OF FREEDOM
Yes, it's all over. Let the wasteland shape your further fate.
Maybe the fundamentals bother you? The game will not go too much into detail! You see, we're definitely going to be able to play a black or interracial couple that fits the same cultural bill as a white couple would. Of course they'd be an exception given the general mindset of the pre-war era, likely looked at irritatedly and secretly mocked at best. I imagine asian people had it worst (Little Yangtze?) due to the overbearing enemy image.
Of course racial and sixual discrimination was still a thing then (even though the government made some attempts to fuel their machinery). Will we see it? No. Of course culture would have demanded homosixuality a sin or just indecent and harmful outside the religious context. I assume that while interracial marriage or black couples who had come to wealth were a thing, same-six marriage was not.
Does that mean you can't play gay or lisbian characters? Of course not. There will be enough freedom within the set story to allow for loopholes.
You want another intro? They won't give you one. Because the intro doesn't matter ultimately. It's all over. Whoever your character was before, whatever explanation you come up with, it will not define the protagonist of the present (the fallen world) who is disillusioned.