Because the Japanese have such freedom. So write flat, katakana, or something. Write Chinese characters if you have them.

Why? I think different people use different words.

But I personally think that most Japanese learners in China should be able to write Chinese characters. A small number can write hiragana.

As for why Beijing is written as katakana, foreign country names or local names are often represented by foreign words in Japanese, so most of them are katakana.

For example, ロンドンニューヨーク, シンガール, ル.

However, China, South Korea, North Korea, Taiwan Province Province and other places use more Chinese characters. The reason should be that these countries are, or used to be, countries that use Chinese characters.