因为没有下载对应的拼音包。
【转】https://askubuntu.com/questions/59356/how-do-i-get-chinese-input-to-work
My system had a similar problem: I was able to choose "Chinese", but it didn't really input any Chinese. The key step missing was to install a specific Chinese input method/package. E.g.:
sudo apt-get install ibus-pinyin
After installing Pinyin in this example, go to System Settings → Language Support → Chinese (Pinyin).
Now it works as expected.
This thread helped me to figure out this solution. It also suggests another input method:
sudo apt-get install ibus-sunpinyin
P.S.
After either of those commands do not forget to restart the IBus daemon
ibus restart