Why should calligraphy be written in traditional Chinese characters instead of simplified Chinese characters?

Because writing traditional Chinese characters is more in line with calligraphy, simplification is a modern thing after all. You can try, find a calligraphy work, pick out the traditional characters and replace them with simplified ones, which will give people a different feeling immediately, and it is also related to the style of calligraphy. You can't just change it. Write as traditionally as possible.