[AnssiH]

My guess is that it is because electronic customs info is now mandatory (e.g. US STOP Act), and the printed labels come from a system that can transmit this info.

With handwritten labels the postal service at the shipping end would have to read them and push the data to an electronic system manually.

In Finland handwritten customs labels are still allowed, but you need to register the customs data online since Dec 2020 (or the cleark can do it at the counter).

[maxgashkov]

This is correct: https://www.post.japanpost.jp/int/ead/index_en.html