[LordKano]

Because they don't want to spend the time.

After too many incorrect pins, there's a time delay before another attempt can be made.

The FBI is also thinking about the next time. They want to be able to take a phone, plug it in and brute force the PIN to gain access.

[centizen]

I have a feeling it's more about setting the precedent, as others have said. There are strategies that can bypass the timeout periods and automatic wiping while bruteforcing, and I'd be surprised if the FBI didn't have at least one rig set up to do it.