Roosevelt and Churchill had been highly at odds about China's world importance. Roosevelt argued that it deserved equal rank with the "Big Three" of Britain, America, and Russia; Churchill retorted that Chang's regime was corrupt and would soon fall. (In the end, both were proved right; China rose to be a world power, but only after Chang's government fell to the Communists.) Roosevelt had convinced Churchill to acquiesce in inviting Chang to the earlier Teheran conference, but Churchill refused to take that as any sort of precedent.

However, for the Yalta Conference in specific, your guess's right: the conference was focused on immediate issues dealing with Europe, where both the Soviets and Western Allies had overrun large areas and were about to join lines, and some form of coordinated policy was urgently needed.

(Source: The Last Lion by Manchester and Reid, a very detailed biography of Winston Churchill.)