Why do we say "Not with a bang but a whimper"?

Meaning:

To end on a muted note - most likely in a situation where one would have expected a more spectacular finish. For example,

"This is the way California ends. Not with a bang, but a whimper. With a failure of leadership so complete, so total, as to leave the state bereft of hope for its future." - Robert Cruickshank

Background:

This expression was coined by T.S. Elliot in his 1925 poem, The Hollow Men, which ends:

This is the way the world ends
This is the way the world ends
This is the way the world ends
Not with a bang but a whimper.

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