The NSA convinced IBM that 56 bits was "enough".
But whereas Lucifer had a key that was 112 bits long, the DES key was shortened to 56 bits at the request of the National Security Agency.
Practical UNIX & Internet Security
In the development of DES, NSA convinced IBM that a reduced key size was sufficient
Data Encryption Standard - Wikipedia
The NSA made two changes to DES: It tweaked the algorithm, and it cut the key size by more than half.