To set a precedent. They had the ability, but wanted to gain access to any phone for any case without having to dip into their budget. Even ones that don't threaten national security.

Because NSLs are very tightly constrained. They're only for telecoms metadata and subscriber information, and they're only for national security use.

There almost certainly were NSLs served on the shooters' phone companies---about the most reasonable use of them one can imagine. They would not compel Apple to do a thing about a phone held by its owner, the City of San Bernadino.

Because they knew the phone wasn't important from day one and they decided to play a political game.

I said a month ago or more that there are companies that do this and the FBI should just ask one. And moreover that these companies were beating down the FBI's door wanting to do it to prove themselves.

NSL only makes it so Apple can't talk about it. It doesn't make it so they have to make a new operating system just because the FBI told them to. There wasn't an easy way to create the precedent of forcing Apple to do what they wanted without it being public.