Technology

Sure, the internet is more practical, and the odds of being caught in the time required to execute a decent strike plan, even one as vague as: "we're going to Amerika and we're going to hit 50 high profile targets on July 4th, one in every state" (Dear NSA analyst, this is entirely hypothetical) so your agents spread to the field and start assessing from the ground the highest impact targets attainable with their resources, extensive back and forth from the field to central command daily for 90 days of prep, but it's being carried out on 270 different active social media channels as innocuous looking photo exchanges with 540 pre-arranged algorithms hiding the messages in the noise of the image bits. Chances of security agencies picking this up from the communication itself? About 100x less than them noticing 50 teams of activists deployed to 50 states at roughly the same time, even if they never communicate anything.

HF (more often called shortwave) is well suited for the numbers game. A deep cover agent lying in wait, potentially for years. Only "tell" is their odd habit of listening to the radio most nights. All they're waiting for is a binary message: if you hear the sequence 3 17 22 you are to make contact for further instructions. That message may come at any time, or may not come for a decade. These days, you would make your contact for further instructions via internet, and sure, it would be more practical to hide the "make contact" signal in the internet too, but shortwave is a longstanding tech with known operating parameters.