The premise of the question is wrong. It assumes, that FBI actually failed to crack an iPhone, but there is no evidence for that (aside from statements of US officials).
More importantly, if FBI, CIA or your uncle could get access to all data on iPhone with just Apple's consent, terrorists could have done the same.
Take right Apple employees hostage, or hack into their computers, and you get access to any iPhone in the world.
That does not sound "hard to crack" at all.
Some people have expressed opinion, that this answer lacks sufficient technical details (not sure why, the question as asked does not mention "technical details"). To illustrate the point above, let's add some technical details.
Suppose, that you have encrypted an USB flash drive with some well-known encryption algorithm. By doing so you ensure (assuming that algorithm is secure), that nobody can access the information within without your personal consent (we are ignoring the non-technical details, remember?)
On other hand, if you encrypt your iPhone with same encryption algorithm, the people, who may give consent to decrypt it are: you and unspecified number of strangers (Apple employees and anyone who they share the key with). This makes for a weaker encryption scheme, since the "key" is forcibly shared amongst unspecified number of people and any of them can obtain your information without consent of others.
As such, based on technical consideration above, iPhone is (on average) less secure that an encrypted USB thumb stick.