1) Globalization has led to larger companies. The larger the company, the higher the pay you'd expect for the CEO (value of each decision is greater).
2) Technology has allowed margins to improve via cheaper cost to scale... economies of scale etc.
3) As another commenter noted, the impact of a CEO is relatively more than in the past, due to advancing technology. More social media presence, more effective means of communication across the company and so on, meaning their decisions are able to effect change more quickly.
4) Globalization has led to reduced wages at the bottom. Unskilled labor is paid less than ever due to that work being outsourced to China, Mexico etc. Some of the change may be more of a story of effective wages falling for the lowest worker.
5) Public CEOs are compensated mostly in stock, and interest rates continue to get lower and lower. Government has been much more aggressive with stimulus than in the past. This has juiced stock valuations.
I'd be interested in seeing how the gap has changed in "like-for-like" businesses. e.g. a 10 person construction company in 1989 vs today.