I believe the answer is very simple - diet and air pollution.
Human jaws are designed to work hard, grinding and chewing at rough, semi-prepared food. Burger and mash just isn't the same!
Developing jaws, that are not exercised much, do not grow as large or as strong as they were designed to. Because of this, the teeth become crowded.
Air pollution, which never existed until recently, irritates the air passages, causing asthma (another uniquely modern complaint) and mouth breathing.
This, in turn, leads to the developing jaw being held open and therefore growing downwards, rather than forwards. This gives rise to that other classic type of malocclusion - the "goofy look", with a recessive lower jaw.
So that is why it happens. Sadly there is nothing much we can do about this - I can't see children being keen to give up their burger buns for coarse, stone ground, unleavened bread!
This does, however, help to explain why non extraction / arch development treatment works so well.
In allowing the jaws to grow to their full potential, space is created for the teeth to grow naturally straight, without the need to take out adult teeth.