Mainly because most threads are FO3 vs FNV and its get very old.
Just the same as Morrowind vs. Oblivion threads get real old real quick.
Fallout 3 has been out for a couple of years now and most, if not all of it has already been posted to death, and New Vegas never quite took off the way many (I at least) had hoped it would. Also not to mention that the Skyrim forums are svcking the life out of what's left, even though most of what's posted in their is just people re posting the same thing over and over.
Once Skyrim is finally released then life will return to it's normal chaotic self here on the forums; and then Fallout 4 will be on the horizon, so that's something else to look forward to.