This is definitely not a scientific study, but I suspect that the media outlets that accept contributed articles are shrinking. The print ones, anyway. I consult to a well known computer magazine whose print issues have fallen from 12 a year to 6, and may go lower. We've gone online, but still. Other competitors have either fallen by the wayside or are doing their impression of the Incredible Shrinking Magazine.
It's advertisers of course, or the lack thereof. You can't just increase editorial when your ads are shrinking because you still have to afford to send the darned thing out. And that means a smaller magazine.
I'd be very interested in getting comments from media relations, PR and marketing folks. In high tech B2B, is it harder to place contributed content? Easier? About the same?