As they say on the website; "Many citizens have devices with USB ports and SD card slots", but more citizens have just eyes and no USB ports/SD card slots, so why not send balloons filled with flyers instead, to reach even more people?
As far as I recall their answer was that a USB drive can store much more. Video, entire books, etc. However leaflets are also done.
Personally I don't know if it's worth the risk knowing that their local OS ("Red Star OS") can fingerprint files. But I guess they know what they're doing.
Most computers in various places like hospitals, veterinarians, shops, gyms and more all have exposed USB ports where it'd be trivial to plug something in when the clerk/assistant is looking the other way, just as another example.