one of the main reasons (if not the main reason) is the huge L2 cache of the c2ds. superpi is a very cache-sensitive application. and intel's cache interface has always been superior to AMD's. the c2d's finally are able to make use of it.
the slowest c2d on the list has 4mb of L2 cache.
my 5000 has 1mb.
that is a big part of why the c2d's are so much faster at superpi. there aren't many applications that the c2ds pull that far ahead, the ones they do, usually are cache-based like superpi.
and just to add on.. lets take a poll here on TF and ask how many people have fried their chips overclocking... it'll probably be a small percent.. like 1...