If you want to know what really happened to the big block Nova, and to the rest of the low production musclecars, COPOs etc, read this clip. It is from John Delorean's book, "On a clear day you can see General Motors". You might want to drink a big cup of coffee first though, this could put you right to sleep. I read the whole book a few years ago, and I can tell you that it was the dullest book I ever read. The second clip is from the good old days, taken from the 1969 Nova brochure. Quite a difference.
All of the Musclecars probably would have died by 1973 even without this deproliferation program, but I think that this shows why they were gone in in 1971 instead of 1973.
http://www.sonic.net/bbdon/free/depro2.jpg
http://www.sonic.net/bbdon/free/depro1.jpg