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
But right now, it's pretty easy to use end-to-end encrypted forms of communication, so why still use sms?
Knowing the suspects and having limited resources, seems to me a argument for listening into communication, not an argument against it.