From what I know of other cultures, the more appropriate question might be, "why do Americans resent upward mobility less than other cultures do?"

See:

In the rest of the English-speaking world, 'Tall poppy syndrome':

http://en.wikipedia.org/wiki/Tall_poppy_syndrome

In Japan, "the stake that sticks up gets hammered down" (though this may be more about nonconformity than success):

http://en.wikipedia.org/wiki/Japanese_proverbs#Sayings

In Scandinavia, 'Jante Law':

http://en.wikipedia.org/wiki/Jante_Law

(American individualism, materialism, frontier-mythology, immigrant-mythology, and rootlessness are all potential explanations for a warmer appreciation of the idea of the 'self-made success'.)