The Chinese government only entered the war on 14th August 1917, nearly three years after the Japanese did. Chinese laborers who went before that time were either contract workers or residents of the enclaves of another colonial power that was a belligerent . See also here. Because the country itself and its government were not at war with Germany at that moment in 1914-1915, the clearing of the enclave fell to Japan. China did regain the Shantung holdings of Germany in the early 1920s, but Qingdao remained out of their hands.

In addition to what /u/khosikulu has said, it also bears noting that in when Japan did opt for belligerency in September 1914, it was still very unclear as to which side was going to win the war or even how long it would last. The Japanese government itself was divided as to whether or not to help Britain by invoking the Anglo-Japanese alliance since many of the officers in the IJA had a decidedly pro-German bent and felt Germany would be the winner in this conflict. Neither the Central Powers nor the Entente sending clear signals about extending the war to the colonies in 1914. British officials in the Foreign and Colonial Offices were actually taken aback by the Japanese offer to take Germany's Pacific colonies. British officialdom expected the IJN to assist chasing German cruisers in the Pacific, not a land-invasion of Qingdao. Deeper Japanese involvement in the war, especially a land-invasion like Qingdao, would mean Japan would demand territorial concessions for the Japanese blood shed. British Foreign Secretary Edward Grey informed Britain's ambassador to Japan that naturally such concessions would come at China's expense. While the British government countenanced this fact of Realpolitik, they did not relish it, especially since advancing Japanese power in the Pacific was a policy that the American State Department opposed. The governments of both Australia and New Zealand were also alarmed by the growth of Japanese power and Japan's entry in the war added more complications to the already anemic plans to get the Pacific and Indian Dominions involved in the war in East Asia. The involvement of a token Indian and British force in Qingdao siege itself caused a minor row inside the British military as the force's commander, Colonel Nathaniel Barnardiston, asked that it be given independent status because he did not want to be subordinate to an Asian military power. Although the British government denied Barnarditson's request, it does demonstrate the significant hurdles Japan's belligerency created.

But what Japan had in 1914, and what Yuan Shikai's government clearly lacked, was a large modern military and a relatively stable governmental structure. When a consensus among Japanese policymakers and the aging Genro formed, Japan could act quickly and force events to their advantage. The IJN already had in its possession existing plans for an invasion of Qingdao and expansion in both the Central Pacific and in China satiated the strategic desires of both Japanese services. For the government in Beijing, war with Germany in 1914 was a risk that would not pay off if Germany prevailed and it was unsure if the Entente would honor any seizure of a European territory in China as it would set a dangerous precedent. For Japan, these were calculated risks it was willing to take because it had the capacity to hold these territories.