They didn't have a challenging time, they just paid some hackers to do it[0].

What they wanted was to set a legal precedent[1] which would allow any law enforcement agency to force manufacturers to subvert the security of their devices on demand.

[0]http://www.reuters.com/article/us-apple-encryption-fbi-idUSK...

[1]https://www.theguardian.com/technology/2016/feb/25/fbi-direc...