One explanation would be because of the cheaper labor and favorable legal frameworks the corporation can grow much larger today. Pushing the pay of the top level positions to rise even higher. You could look at C-level positions 50 years ago as a mid-level manager today in terms of managed organization size and economic impact.
In the big 5 that I work for, MBA's become product managers and make less than programmers (per years in industry). They then have to compete for more than a decade (with programmers going into management), for a shot at even VP level.
Guess what, there are some VP's that have 5 more layers of VP, Senior VP's... before C suite.
You want to incentive people to work hard at their jobs to get promoted to the next level so you need to constantly pay them more. When you have 10 layers in your company and people at the bottom are making 6 figures (for software engineers, easily the case), the top level employees are going to make a lot of money.
