Why is Western punctuation used in Chinese writing?
I have found answers in Chinese Wikipedia:  


Classical Chinese has a punctuation system of its own. However, this system is rarely used. The method to read Classical Chinese without punctuation was taught in elementary schools in the past, but it was still too difficult for common people to understand. You may know that Chinese is an isolating language, which means morphemes are quite sequence-sensitive. Even a simple phrase can have quite a few different meanings. In Modern Chinese, this appears even more often.
Examples:
是你 (It) is you
是你。 It is you.
是你？ Is it you?
哼！是你？ Hmph! Is it you? (contempt)
During the late 1910s, there is a movement called New Culture Movement, and a major goal of it is to teach everyone to read and write. The Classical Chinese was too hard for people to read, and lacking punctuation is one of the difficulties. On 29 November, 1919, Ma Yuzao and other 5 person submitted a proposal of calling for using modern punctuation system, which mixed classical and Western punctuations (according to Guan Xihua, 2002). Outline of History of Chinese Philosophy by Hu Shi is the first work applying this punctuation system.  
The reason of adapting a "Western" punctuation system is to make reading texts easier, especially for those who is learning reading for the first time. Since the using of new punctuation system, Baihuawen, or Modern Chinese was popularized quite rapidly.