Besides most empires never really "notice" their fall, the SU is an exception.That is a gross generalization and not really correct.Same reason the British Empire Collapsed.
Eventually, all empires collapse. Because the imperial nation gets tired of ruling and the subject people get tired of being ruled.
To say the USSR collapsed because of "Communism' is just as foolish as saying the British Empire collapsed because of Capitalism.
http://people.uncw.edu/kozloffm/glubb.pdf
yup....200-250 years .....nothing to get nervous about!
~S~