Keep in mind that strong security is similar to a wall safe. The only two ways to penetrate strong security is either with the key (combination) or through brute force. Even if the hacker has the blueprints to the safe or the software source code for the encryption it doesn't help with the attack.
Hard Come, Easy GoTrust is hard to earn and easy to lose. Apple's stance against the federal government stems from the NSA's PRISM program. The Daily Beast article points out, "It wasn’t until after the revelations of former NSA contractor Edward Snowden that Apple began to position itself so forcefully as a guardian of privacy protection in the face of a vast government surveillance apparatus."
While federal officials speculate that access to the San Bernardino shooter's iPhone data might be a national security issue, that's not enough to warrant the creation of a backdoor. Keeping America safe is not the oath that elected officials or military officers take. Rather, their oath is to support and defend the Constitution. It is not a stretch to see how America could be much safer by curtailing our Bill of Rights. Saudi Arabia is a safe country to live in, with low crime.
Update 21 Feb 2016: Another point to consider is that Apple Inc. is a multi-national corporation located in different countries and jurisdictions. If one argues that Apple has to develop a special iOS version for the US government, in this single case, in the name of terrorism, then should Apple also develop special iOS versions for, say, France or China if requested? As the New Yorker’s Amy Davidson writes, “If it [the government] can tell Apple, which has been accused of no wrongdoing, to sit down and write a custom operating system for it, what else could it do?”