Look at the existing cases of terrorism. All of them used unencrypted forms of communication: shared Gmail account, phone calls, even good old SMS. The issue is not that police can't see what those people are doing, none of the perpretators are unknown; it's that they (and the judiciary branch) don't have enough resources to really tackle them.
I'm not exactly an expert on the issue, but I could imagine that they use sms because that's the only available form of communication on an anonymous $10 dumbphone, and any computer connected to the internet can connect to a webmail.
Listening to more communications is exactly a resource problem: it isn't exactly cheap to build and maintain the infrastructure to gather and analyze more. Moreover in the context of the link, while I'm not a fan of conversations being listened on by default, I certainly don't want resources be spent so that encryption can be reduced
