I've heard numerous "theories" on this. The one that seems the most believable to me is the following:

• The recent advances to healthcare along with working conditions becoming much safer for people who aren't physically strong has led to an increase in weak men surviving whereas in the past, they would have died. This leads to the overall makeup of society being biased toward having a lot more weak men than in the past. Also, the socialization of everyone allows the weakest to survive but in more free market societies (less socialism), the weak simply die off.

I first heard this theory proposed by a scientist who believed he essentially identified a "beta" and "alpha" gene. He called it something else, but it's a gene where people would be strong or a "warrior-like", and a gene where they wouldn't. A philosopher might call it a slave vs. a master gene. In any case, he believed that in recent history, the alpha gene always dominated the beta gene but our recent advantages in technology, namely in healthcare and sanitation has led to people with the beta gene flourishing, whereas in the past, most of these people would have been the ones who died off and thus we would have always had an excess of alpha genes compared to beta genes in society.

So now we have way more betas compared to warriors and society is structured such that our governments have immense power over individual men thanks to weapons technology and communications technology being the way that it is.

Essentially, our warriors are still here but they are oppressed thanks to the governments immense power and being outnumbered by the beta/weak men who in the past would have never had the numbers they have today thanks to advances in healthcare and sanitation as well as jobs becoming much safer and not requiring as much physical strength.