Another point worth mentioning which was covered very recently here is that it's difficult to get agreement on the rights to fights in terms of releasing DVD's etc, I presume the same would be said for showing older fights on TV.
As I see it from a personal view, what's the loss in releasing say a box set of fights from each major promotor each year, even if it doesn't pull in a lot of $$ something is better than nothing.

Yet another contentious area where managers and fighters probably price themselves out of making these deal viable.