Japan had better strategies and were technologically more advanced. China during that time, had also just gotten out of the war with the British and were weak
Here are some reasons take your pick. Good luck! :)
Japan had modernized weapons
Corruption of the Chinese government-officials embezzled military funds.
Chinese army and navy had little preparation for the war
Japan had better war tactics and strategies
Chinese just finished the War of Opium and were weak
The majority of China’s massive population was not educated
