My understanding is that the iPhone 5C is not _simple_ to brute force as sold from the store. The FBI is asking Apple to weaken the software in this particular one enough for them to be able to brute-force by creating custom software that would allow them to try millions of passcode combinations rapidly.

In the past law enforcement could use their own tools and Apple didn't have any legal way to say "it is beyond our ability to break it so we can't help you" anyway. After their name showed up on that slide in the Prism leak without their cooperation (meaning they had been stepped around by the FBI. Some of the earlier companies had willingly volunteered data), they stepped up their game and deployed end-to-end encryption and secure enclave to have the ability to say 'we can't help' when forced to.

This technique wouldn't be possible on the iPhone 6 due to the encryption keys being in the hardware secure enclave but they are putting their foot down now so that a legal precedent isn't established forcing them to weaken other models too. That's my understanding of it right now.