Why is the maximum message size of the SHA-1 hash function (2^64) - 1 bits?

Question

The title of the question describes everything.
As described on http://en.wikipedia.org/wiki/SHA-1#Comparison_of_SHA_functions
Why is the maximum message size for the SHA-1 hash function limited to (2^64) - 1 bits?

Answer 1

Since in hash finalization the unsigned 64-bit message length is appended to the last block and hashed.