The FBI needs a plausible chain of custody regarding this iPhone - every single thing that has been done to it, by whom, when, and how it was done - if it wants to use it for prosecutions in court, or to obtain warrants for search, seizure or wiretaps from a court.
This is why the FBI (DHS) needs "Apple assistance" to open it, unless they want to either disclose in court records that they successfully surreptitiously opened it, or have a big redacted pubic record regarding the opening process which is as good as saying they cracked it.
Apple doesn't want that either, because the conclusion is that Apple iPhones are hackable by the US government without any help from Apple. As such, Apple will eventually "open" it for them, but will kick the can a while publicly to save their image and stock value.