The real reason is the millions of other phones out there. If there was any indication that this couple was acting as the vanguard of a large number of others acting in a similar way what exactly do you think the chances are that that data would be left on a phone when they took the effort of destroying their other phone?

This is the prelude to a large number of fishing expeditions and it seems the FBI made this particular effort possible by first ordering a third party to make it impossible to reach the data in any other way. If they actually cared about that data you'd think they had contacted Apple right away about the best possible way to get it out without bringing this to a head over a terrorist case where lots of panicky people would make the wrong decision out of fear.

If you really are concerned about your privacy: don't store your important stuff in a phone or a computer for that matter.

Ironically, terrorists on a suicide mission don't need ironclad encryption. They just need to stay ahead of the law long enough to do their deed and to make sure the data flows in only one direction through their organization without leaving a source address for any transmissions. So for actual terrorists that are even remotely looking at their operational security they could be transmitting at the same level within their cell in plain text using a few silly code words and you'd only know what they were up to when the ambulances arrived.