When I was reading a Japanese book at an airport in overseas, a foreigner asked me what language was written in my book. After I answered this book was written in Japanese, he said that he had asked me because vertical writings were rare. Thus I noticed that Japanese written vertically was relatively rare.
According to Wikipedia, the majority of languages in the world are written horizontally. Some East Asian languages including Korean and Japanese used to be written vertically by the influence of Chinese. Nowadays, Chinese and Korean are usually written horizontally. Now, Japanese is also written partially horizontally in cases of books about mathematics, science, music, movie, game and foreign languages due to the convenience. Internet articles are also written horizontally.
Still, most of the Japanese writings such as newspaper, novels, comics, and laws are written vertically. It is simply a cultural custom, but some foreigner may feel strange about it and it may be difficult for them to become familiar with this language at first. I hope that they will get used to it soon when they start studying Japanese.