People tend to overestimate the cultural level of Southern Italy criminals. They are men dedicated to a cult of physical and psychological violence, nothing more. For anything more complicated than that, they have henchmen, and they just keep them in check with violence.
In particular, regarding communications, phone tapping taught them that anything electronic, anything they don't fully control, can be used by authorities to spy on them. Very simple people learn very simple lessons: "it doesn't matter what technology you pick, you cannot trust it". Their henchmen can use what they prefer to talk among themselves, so they might use other stuff; but the very top people will stay far away from anything that might burn them.
That should read: "burying scraps in Sicilian soil"
The messages never travel outside of the country, which may bolster a case that no other country may have jurisdiction in helping to track down the mafia members.
Recent Intel leaks have shown us that every government in the world has their fingers tapping all of the telecom links in the world. When we talk about using farm land as a messaging platform, the playing field is a bit more even.
The government doesn't have cameras hiding under every rock, or soldiers posted every 5 feet across the land. Imaging satellites perhaps may not be able to be tasked full-time for years to monitor all of Sicily to the required resolution for the purposes of finding one man, yet.
Balled-up pieces of paper are very much off-the-grid.