Honestly? Women. You started seeing a shift into double income families in the 70's. And it grew exponentially from there. It's not the fault of women for creating the difficulty of getting decent jobs.

It's simply the effect that in America, at least, women joining the workforce doubled the amount of available workers. It's why incomes haven't risen proportionally to inflation. There are simply so many workers available now that both genders are working in full force.

That's one fraction of the cause job placement becoming increasingly more difficult. Every person in the US is told to achieve success, a college degree is needed. So we do have an educated workforce, but throwing more readily qualified workers into an industry doesn't cause that industry to grow substantially. So we have an ample supply of skilled workers, but not a huge demand in the industries needed to support those workers.

Next is automation and manufacturing. As a business owner, why have 10 workers when they can have one worker oversee a few dozen automated machines? If a process requires repetition, it will get automated eventually. Repetitive professions slowly die off and then we're left skilled workers that, more than likely, are not flexible enough to move to another trade rapidly.

Now, a smarter person than me could provide a bunch of links into this. I'm probably even wrong on this. I believe these are a few contributing factors though.