I think the instantaneous nature of chat rooms coupled with the fact that they're accessible to all leads to this decline. Forums have a degree of premeditation, making them (generally) more lucid than chat rooms. Even so, they often degenerate into flaming.
In the case of the chat room, it has its moments, but a lot of the chat is taken up by solicitations for camera sex. Unfortunately, this means that those who actually want to chat dissappear into IMs with those people they know, to the detriment of the room as a whole. This is my rather benevolent interpretation, anyway. It's quite possible that the majority are masturbating frantically on camera with each other.