There are probably some specific factors--like the rise in healthcare benefit costs. But, overall, there's just an increase in specialization throughout the economy. As you go back in time, you just generally had fewer services companies that were available to fill every need that was tangential to the main focus of a given company.

To give one example that goes back a little further, companies used to handle the whole payroll process themselves. Now they use ADP or whoever.

There's also been a lot of automation/self-service of mundane tasks. You don't really have the stereotypical "starting the mailroom" job any longer nor do you have the steno pool.