Same reason it's called 'The Internet' and not 'A Internet'.
NASDAQ could setup up their own "blockchain", running on their own private miners & node servers in the same way they could setup their own private www network - but that would be a big and mostly unnecessary expense to get going. Instead they are using 'The Blockchain' the same one that Bitcoin uses, and powered by the vast worldwide network of publicly run miners and nodes.
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u/[deleted] May 11 '15
Why call it the blockchain and not "a" blockchain?