[jsoc815]

Two reasons come to mind:

1) Contract negotiations;

2) It's said because they are the face of the organization (and hence, whipping boy/girl) that their jobs are less secure than those of a rank-and-file employee. So, to incentivize someone to take/keep the job and cope w/the increased risks, they must be paid more.

*There also an age old story about a compensation study that was done by, IIRC, McKinsey that is said to be responsible for the compensation inflation that has appeared over that last few decades or so. If you search around on the web, you can probably nail down references to or specifics of it.