There are probably several reasons for this.

Off the top of my head:
-women entering higher education and (later) the workforce in higher numbers, which obviously enhances the pool of potential employees for companies
-the slow decline of several labor-intensive industries (e.g. coal mining) industries
-automation, which enables companies to achieve the same output with fewer human workers
-downward pressure due to actual outsourcing of jobs to low-cost countries in Asia (I´d argue that even the threat of it keeps wages down)
-less powerful unions compared to a few decades ago in most countries
...