Because they were useful for targeted surveillance? Not that I agree with the means or the scope, but there's an above board explanation for the desire to get the keys . Suppose you have a handful of phones in Pakistan or Iran you need access to very covertly (e.g. some rogue guy in the ISI where getting caught snooping has major consequences). The least risky way to access his communications is to get the keys. The least risky way to do that is to get them from the broadest source possible(to obscure who you're really interest in) and the one most removed from your target. So there's a legit reason to want the keys, even if your only targeting a few legit targets.

But the means of doing so is truly questionable, even given all their assertions about trust us and we don't look at everyones stuff.