Snail mail, even better certified mail, is a de facto standard that can reach everyone everywhere. Also, it make sense that the most widespread and solid communication channel is still considered the official one
Here in Italy you can also use certified email instead of snail mail, but you need to purchase and pay (yearly) for an account. So snail mail still ubiquitous and solid. Certified email better, faster, if you can afford it (pretty cheap, but require an active action from you, to get it).