Because English is the world's standard communication for all things aviation.
Latin alphabet.
Romanization of Chinese is the use of the Latin alphabet to write Chinese. Chinese uses a logographic script and its characters do not represent phonemes directly. There have been many systems using Roman characters to represent Chinese throughout history. Linguist Daniel Kane recalls, “It used to be said that sinologists had to be like musicians, who might compose in one key and readily transcribe into other keys.”[1] The dominant international standard for Putonghua since about 1982 has been Hanyu Pinyin. Other well-known systems include Wade–Giles (Mandarin) and Yale Romanization (Mandarin and Cantonese).
There are many uses for Chinese Romanization. Most broadly, it is used to provide a useful way for foreigners who are not skilled at recognizing Chinese script to read and recognize Chinese. It can also be helpful for clarifying pronunciation among Chinese speakers who speak mutually unintelligible Chinese varieties. Romanization facilitates entering characters on standard keyboards such as QWERTY. Chinese dictionaries have complex and competing sorting rules for characters and romanization systems simplify the problem by listing characters in their Latin form alphabetically.