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.

from [Practical UNIX & Internet Security](http://www.diablotin.com/librairie/networking/puis/ch06_04.htm)

> In the development of DES, NSA convinced IBM that a reduced key size was sufficient

from [Data Encryption Standard - Wikipedia](https://en.wikipedia.org/wiki/Data_Encryption_Standard#NSA.27s_involvement_in_the_design)

> The NSA made two changes to DES: It tweaked the algorithm, and it cut the key size by more than half.

from [The Legacy of DES - Bruce Schneier](https://www.schneier.com/blog/archives/2004/10/the_legacy_of_d.html)